EPiServer CMS 6 R2 to 7.5 upgrade issue

We have upgraded our EPiServer CMS 6 R2 site to EPiServer CMS 7.5. After the upgrade I encounter the issue below when starting the site:

Can't find schema System.Collections.Generic.List`1[EPiServer.WorkflowFoundation.Workflows.ApproverItem] to deserialize against

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Runtime.Serialization.SerializationException: Can't find schema System.Collections.Generic.List`1[EPiServer.WorkflowFoundation.Workflows.ApproverItem] to deserialize against

Source Error:

[No relevant source lines]

Source File: c:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\root\b2cfd15c\2258024c\App_global.asax.0.cs Line: 0

Stack Trace:

[SerializationException: Can't find schema System.Collections.Generic.List`1[EPiServer.WorkflowFoundation.Workflows.ApproverItem] to deserialize against]
   EPiServer.Implementation.Serialization.ObjectReader.LoadObject() +542
   EPiServer.Implementation.Serialization.ObjectReader.LoadArray() +225
   EPiServer.Implementation.Serialization.ObjectReader.LoadObjectForType(Type type, ISchema serializedSchema, Type[] genericTypeParameters) +387
   EPiServer.Implementation.Serialization.ObjectReader.LoadObject() +369
   EPiServer.Implementation.Serialization.ObjectReader.LoadObjectForType(Type type, ISchema serializedSchema, Type[] genericTypeParameters) +458
   EPiServer.Implementation.Serialization.ObjectReader.LoadObject() +369
   EPiServer.Implementation.SqlObjectStore.Load(Object id) +540

[ObjectStoreException: Exception on Load of 1a55c7e2-017a-441a-b208-7708d02f3858 in object store localdatabase.]
   EPiServer.Implementation.SqlObjectStore.Load(Object id) +1123
   EPiServer.Implementation.SqlObjectStore.RelatedItemsFrom(Object fromId, Boolean includeSubclasses, LoadMethod loadMethod) +762

[ObjectStoreException: Exception on Load of db0090ad-b10d-40ac-9407-cdc173815641 in object store localdatabase.]
   EPiServer.Implementation.SqlObjectStore.RelatedItemsFrom(Object fromId, Boolean includeSubclasses, LoadMethod loadMethod) +1548
   EPiServer.Implementation.SqlObjectStore.RelatedItemsFrom(Object fromId, LoadMethod loadMethod) +27
   EPiServer.Implementation.CacheObjectStore.RelatedItems(Object id, Boolean isFromRelation, LoadMethod loadMethod) +714
   EPiServer.Implementation.CacheObjectStore.RelatedItemsFrom(Object fromId) +108
   EPiServer.WorkflowFoundation.StorageProviders.ObjectStoreStorageProvider.LoadDefinitions() +825
   EPiServer.WorkflowFoundation.DefinitionHandler.LoadDefinitions() +87
   EPiServer.WorkflowFoundation.DefinitionHandler.get_WorkflowDefinitions() +121
   EPiServer.WorkflowFoundation.DefinitionHandler.ReloadDefinitions() +128
   EPiServer.WorkflowFoundation.AspNetWorkflowManager.CreateDefinitionHandler() +312
   EPiServer.WorkflowFoundation.AspNetWorkflowManager.Initialize(Boolean lazyLoading) +1869
   EPiServer.WorkflowFoundation.WorkflowSystem.StaticInitialization(WorkflowSettingsElement configElement) +927
   EPiServer.Framework.Initialization.ModuleNode.Execute(Action a, String key) +64
   EPiServer.Framework.Initialization.InitializationEngine.InitializeModules() +719
   EPiServer.Framework.Initialization.InitializationEngine.Initialize(HostType hostType) +168
   EPiServer.Framework.Initialization.InitializationModule.Initialize(HostType hostType) +285
   EPiServer.Framework.Initialization.InitializationModule.FrameworkInitialization(HostType hostType) +104
   EPiServer.Global..ctor() +110
   ASP.global_asax..ctor() in c:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\root\b2cfd15c\2258024c\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) +159
   System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, StackCrawlMark& stackMark) +256
   System.Activator.CreateInstance(Type type, Boolean nonPublic) +127
   System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes, StackCrawlMark& stackMark) +14259449
   System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes) +200
   System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture) +28
   System.Web.HttpRuntime.CreateNonPublicInstance(Type type, Object[] args) +83
   System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context) +312
   System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext) +475

[HttpException (0x80004005): Exception has been thrown by the target of an invocation.]
   System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +12979668
   System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +159
   System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +12819261