That class was moved to EPiServer.Cms.AspNet.dll in CMS 11, so I suppose one of the assemblies has reference to it. That assembly would need to be recompiled.

What is your full stacktrace? 

This is my full stack trace

[TypeLoadException: Could not load type 'EPiServer.PlugIn.GuiPlugInAttribute' from assembly 'EPiServer, Version=11.2.1.0, Culture=neutral, PublicKeyToken=8fe83dea738b45b7'.]
System.ModuleHandle.ResolveType(RuntimeModule module, Int32 typeToken, IntPtr* typeInstArgs, Int32 typeInstCount, IntPtr* methodInstArgs, Int32 methodInstCount, ObjectHandleOnStack type) +0
System.ModuleHandle.ResolveTypeHandleInternal(RuntimeModule module, Int32 typeToken, RuntimeTypeHandle[] typeInstantiationContext, RuntimeTypeHandle[] methodInstantiationContext) +145
System.Reflection.RuntimeModule.ResolveType(Int32 metadataToken, Type[] genericTypeArguments, Type[] genericMethodArguments) +158
System.Reflection.CustomAttribute.FilterCustomAttributeRecord(CustomAttributeRecord caRecord, MetadataImport scope, Assembly& lastAptcaOkAssembly, RuntimeModule decoratedModule, MetadataToken decoratedToken, RuntimeType attributeFilterType, Boolean mustBeInheritable, Object[] attributes, IList derivedAttributes, RuntimeType& attributeType, IRuntimeMethodInfo& ctor, Boolean& ctorHasParameters, Boolean& isVarArg) +91
System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeModule decoratedModule, Int32 decoratedMetadataToken, Int32 pcaCount, RuntimeType attributeFilterType, Boolean mustBeInheritable, IList derivedAttributes, Boolean isDecoratedTargetSecurityTransparent) +466
System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeType type, RuntimeType caType, Boolean inherit) +160
System.RuntimeType.GetCustomAttributes(Boolean inherit) +43
EPiServer.Framework.Initialization.<>c.<ScanAssemblies>b__46_0(Type t) +56
System.Linq.WhereEnumerableIterator`1.MoveNext() +90
System.Linq.<DistinctIterator>d__63`1.MoveNext() +64
System.Linq.WhereSelectEnumerableIterator`2.MoveNext() +190
System.Linq.<CastIterator>d__94`1.MoveNext() +54
System.Collections.Generic.List`1..ctor(IEnumerable`1 collection) +333
System.Linq.Enumerable.ToList(IEnumerable`1 source) +58
EPiServer.Framework.Initialization.InitializationEngine.ScanAssemblies() +460
EPiServer.Framework.Initialization.InitializationEngine.Initialize() +56
EPiServer.Framework.Initialization.<>c.<FrameworkInitialization>b__7_0(InitializationEngine e) +36
EPiServer.Framework.Initialization.InitializationModule.EngineExecute(HostType hostType, Action`1 engineAction) +591
EPiServer.Framework.Initialization.InitializationModule.FrameworkInitialization(HostType hostType) +216
EPiServer.Global..ctor() +83
ProjectDev.EPiServerApplication..ctor() +42
ASP.global_asax..ctor() in c:\Users\Robert Vandermeer\AppData\Local\Temp\Temporary ASP.NET Files\root\cfa79eba\a0ab50a0\App_global.asax.0.cs:0

[TargetInvocationException: Exception has been thrown by the target of an invocation.]
System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean noCheck, Boolean& canBeCached, RuntimeMethodHandleInternal& ctor, Boolean& bNeedSecurityCheck) +0
System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, StackCrawlMark& stackMark) +114
System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, StackCrawlMark& stackMark) +232
System.Activator.CreateInstance(Type type, Boolean nonPublic) +83
System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes, StackCrawlMark& stackMark) +1088
System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes) +128
System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture) +20
System.Web.HttpRuntime.CreateNonPublicInstance(Type type, Object[] args) +60
System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context) +260
System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext) +296

[HttpException (0x80004005): Exception has been thrown by the target of an invocation.]
System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +9963380
System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +101
System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +254

Are you using any third parties assemblies? 

Yes Quan. I was using https://github.com/Geta/Tags

After removing this, the solutions worked fine.

Looks like the The latest version is compiled for .NET 4.5 and EPiServer 10.

Thank you for your answer.

<p>Wiselin, <a href="https://github.com/Geta/Tags/issues/54">there is an issue</a>&nbsp;on Github about this.</p>

Valdis, Okay i see that. I will be watching out for any upate on that issue in github then. thank you for poitning that.

Hey guys,

i have the same problem. I also used Geta Tags en Geta Sitemap, but those are upgraded to be compatible with 11.x as well.. 

All other 3rd party addons are upgraded and i checked are all 11.x compatible because they all updated their packages as well.

I have these installed.

<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="BVN.404Handler" version="11.1.0" targetFramework="net462" />
  <package id="Castle.Core" version="4.2.1" targetFramework="net462" />
  <package id="Castle.Windsor" version="4.1.0" targetFramework="net462" />
  <package id="DotNetZip" version="1.9.1.8" targetFramework="net451" />
  <package id="EntityFramework" version="6.0.0" targetFramework="net451" />
  <package id="EPiServer.CMS" version="11.3.3" targetFramework="net462" />
  <package id="EPiServer.CMS.AspNet" version="11.3.3" targetFramework="net462" />
  <package id="EPiServer.CMS.Core" version="11.3.3" targetFramework="net462" />
  <package id="EPiServer.CMS.TinyMce" version="1.0.0" targetFramework="net462" />
  <package id="EPiServer.CMS.UI" version="11.2.5" targetFramework="net462" />
  <package id="EPiServer.CMS.UI.Core" version="11.2.5" targetFramework="net462" />
  <package id="EPiServer.Find" version="12.7.1" targetFramework="net462" />
  <package id="EPiServer.Find.Cms" version="12.7.1" targetFramework="net462" />
  <package id="EPiServer.Find.Framework" version="12.7.1" targetFramework="net462" />
  <package id="EPiServer.Framework" version="11.3.3" targetFramework="net462" />
  <package id="EPiServer.Framework.AspNet" version="11.3.3" targetFramework="net462" />
  <package id="Episerver.GoogleMapsEditor" version="1.0.11.0" targetFramework="net462" />
  <package id="EPiServer.Logging.Log4Net" version="2.2.1" targetFramework="net462" />
  <package id="EPiServer.Packaging" version="3.4.0" targetFramework="net462" />
  <package id="EPiServer.Packaging.UI" version="3.4.0" targetFramework="net462" />
  <package id="EPiServer.Search" version="9.0.1" targetFramework="net462" />
  <package id="EPiServer.ServiceLocation.StructureMap" version="2.0.0" targetFramework="net462" />
  <package id="EPPlus" version="4.1.0" targetFramework="net452" />
  <package id="Geta.EPi.Extensions" version="2.0.1" targetFramework="net462" />
  <package id="Geta.Net.Extensions" version="1.0.2" targetFramework="net462" />
  <package id="Geta.SEO.Sitemaps" version="2.0.2" targetFramework="net462" />
  <package id="Geta.Tags" version="3.0.1" targetFramework="net462" />
  <package id="log4net" version="2.0.8" targetFramework="net462" />
  <package id="logentries.core" version="2.8.0" targetFramework="net452" />
  <package id="logentries.log4net" version="2.8.0" targetFramework="net452" />
  <package id="Lucene.Net" version="3.0.3" targetFramework="net451" />
  <package id="Microsoft.AspNet.Mvc" version="5.2.3" targetFramework="net452" />
  <package id="Microsoft.AspNet.Providers.Core" version="2.0.0" targetFramework="net451" />
  <package id="Microsoft.AspNet.Razor" version="3.2.3" targetFramework="net452" />
  <package id="Microsoft.AspNet.WebApi.Client" version="5.0.0" targetFramework="net452" />
  <package id="Microsoft.AspNet.WebApi.Core" version="5.0.0" targetFramework="net452" />
  <package id="Microsoft.AspNet.WebApi.WebHost" version="5.0.0" targetFramework="net452" />
  <package id="Microsoft.AspNet.WebPages" version="3.2.3" targetFramework="net452" />
  <package id="Microsoft.CodeDom.Providers.DotNetCompilerPlatform" version="1.0.8" targetFramework="net462" />
  <package id="Microsoft.Net.Compilers" version="2.4.0" targetFramework="net462" developmentDependency="true" />
  <package id="Microsoft.Net.Http" version="2.0.20710.0" targetFramework="net452" />
  <package id="Microsoft.Tpl.Dataflow" version="4.5.24" targetFramework="net452" />
  <package id="Microsoft.VisualStudio.SlowCheetah" version="3.0.61" targetFramework="net452" />
  <package id="Microsoft.Web.Infrastructure" version="1.0.0.0" targetFramework="net451" />
  <package id="Microsoft.WindowsAzure.ConfigurationManager" version="3.1.0" targetFramework="net452" />
  <package id="Newtonsoft.Json" version="10.0.3" targetFramework="net462" />
  <package id="NuGet.Core" version="2.5.0" targetFramework="net451" />
  <package id="PagedList" version="1.17.0.0" targetFramework="net452" />
  <package id="PagedList.Mvc" version="4.5.0.0" targetFramework="net452" />
  <package id="Pri.LongPath" version="1.3.2.0" targetFramework="net452" />
  <package id="SharpZipLib" version="0.86.0" targetFramework="net451" />
  <package id="SimpleOAuth.Net" version="1.0.1" targetFramework="net451" />
  <package id="SiteImprove.EPiServer.Plugin" version="2.2.1" targetFramework="net452" />
  <package id="StructureMap" version="4.5.2" targetFramework="net462" />
  <package id="structuremap.web" version="4.0.0.315" targetFramework="net462" />
  <package id="structuremap.web-signed" version="3.1.6.186" targetFramework="net452" />
  <package id="structuremap-signed" version="3.1.9.463" targetFramework="net452" />
  <package id="System.ComponentModel.Annotations" version="4.4.0" targetFramework="net462" />
  <package id="System.Data.SqlClient" version="4.4.0" targetFramework="net462" />
  <package id="System.Reflection.Emit" version="4.3.0" targetFramework="net462" />
  <package id="System.Reflection.Emit.Lightweight" version="4.3.0" targetFramework="net462" />
  <package id="System.Security.AccessControl" version="4.4.0" targetFramework="net462" />
  <package id="System.Security.Cryptography.Xml" version="4.4.0" targetFramework="net462" />
  <package id="System.Security.Permissions" version="4.4.0" targetFramework="net462" />
  <package id="System.Security.Principal.Windows" version="4.4.0" targetFramework="net462" />
  <package id="System.Threading.AccessControl" version="4.4.0" targetFramework="net462" />
  <package id="TamTam.NuGet.LockHelper" version="1.0.0.5" targetFramework="net452" />
  <package id="TimePeriodLibrary.NET" version="2.0.0" targetFramework="net452" />
</packages>

[TypeLoadException: Could not load type 'EPiServer.PlugIn.GuiPlugInAttribute' from assembly 'EPiServer, Version=11.3.3.0, Culture=neutral, PublicKeyToken=8fe83dea738b45b7'.]
   System.ModuleHandle.ResolveType(RuntimeModule module, Int32 typeToken, IntPtr* typeInstArgs, Int32 typeInstCount, IntPtr* methodInstArgs, Int32 methodInstCount, ObjectHandleOnStack type) +0
   System.ModuleHandle.ResolveTypeHandleInternal(RuntimeModule module, Int32 typeToken, RuntimeTypeHandle[] typeInstantiationContext, RuntimeTypeHandle[] methodInstantiationContext) +191
   System.Reflection.RuntimeModule.ResolveType(Int32 metadataToken, Type[] genericTypeArguments, Type[] genericMethodArguments) +161
   System.Reflection.CustomAttribute.FilterCustomAttributeRecord(CustomAttributeRecord caRecord, MetadataImport scope, Assembly& lastAptcaOkAssembly, RuntimeModule decoratedModule, MetadataToken decoratedToken, RuntimeType attributeFilterType, Boolean mustBeInheritable, Object[] attributes, IList derivedAttributes, RuntimeType& attributeType, IRuntimeMethodInfo& ctor, Boolean& ctorHasParameters, Boolean& isVarArg) +148
   System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeModule decoratedModule, Int32 decoratedMetadataToken, Int32 pcaCount, RuntimeType attributeFilterType, Boolean mustBeInheritable, IList derivedAttributes, Boolean isDecoratedTargetSecurityTransparent) +607
   System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeType type, RuntimeType caType, Boolean inherit) +214
   EPiServer.Framework.Initialization.<>c.<ScanAssemblies>b__46_0(Type t) +31
   System.Linq.WhereEnumerableIterator`1.MoveNext() +123
   System.Linq.<DistinctIterator>d__64`1.MoveNext() +79
   System.Linq.WhereSelectEnumerableIterator`2.MoveNext() +263
   System.Linq.<CastIterator>d__95`1.MoveNext() +74
   System.Collections.Generic.List`1..ctor(IEnumerable`1 collection) +504
   System.Linq.Enumerable.ToList(IEnumerable`1 source) +70
   EPiServer.Framework.Initialization.InitializationEngine.ScanAssemblies() +368
   EPiServer.Framework.Initialization.InitializationEngine.Initialize() +34
   EPiServer.Framework.Initialization.InitializationModule.EngineExecute(HostType hostType, Action`1 engineAction) +420
   EPiServer.Framework.Initialization.InitializationModule.FrameworkInitialization(HostType hostType) +225
   EPiServer.Global..ctor() +103
   HR.Website.EPiServerApplication..ctor() +37
   ASP.global_asax..ctor() +42

[TargetInvocationException: Exception has been thrown by the target of an invocation.]
   System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean noCheck, Boolean& canBeCached, RuntimeMethodHandleInternal& ctor, Boolean& bNeedSecurityCheck) +0
   System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, StackCrawlMark& stackMark) +142
   System.Activator.CreateInstance(Type type, Boolean nonPublic) +105
   System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes, StackCrawlMark& stackMark) +1515
   System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes) +186
   System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture) +27
   System.Web.HttpRuntime.CreateNonPublicInstance(Type type, Object[] args) +79
   System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context) +281
   System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext) +329

[HttpException (0x80004005): Exception has been thrown by the target of an invocation.]
   System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +525
   System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +118
   System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +708

Thats is my error... I really need this to work. I had 2 external DLL's referenced but i have removed those, to see if that was the problem, but still this error. 

What else am i missing?

<p>Nevermind i found the problem:&nbsp; the package&nbsp; SiteImprove.Episerver.Plugin was causing the fail.&nbsp;</p> <p></p> <p>The funny thing is they say it's compatible with &gt;= 10.0.2&nbsp; but that's not true :-)</p> <p>&nbsp;https://nuget.episerver.com/en/OtherPages/Package/?packageId=SiteImprove.EPiServer.Plugin&amp;packageVersion=1.4.9</p> <p></p>