If you open the network tab in browser console, choose the 500 request and open in new tab, what do you see? 

Hi Quan Mai,

I am geeting error ERROR EPiServer.Global: 1.2.5 Unhandled exception in ASP.NET
System.ArgumentException: Unable to find a module by assembly 'EPiServer.TinyMCESpellChecker, Version=1.0.1.75, Culture=neutral, PublicKeyToken=null'
Parameter name: moduleAssembly.

Error Description :

ERROR EPiServer.Global: 1.2.5 Unhandled exception in ASP.NET
System.ArgumentException: Unable to find a module by assembly 'EPiServer.TinyMCESpellChecker, Version=1.0.1.75, Culture=neutral, PublicKeyToken=null'
Parameter name: moduleAssembly
at EPiServer.Shell.Paths.ToResource(Assembly moduleAssembly, String moduleRelativeResourcePath)
at EPiServer.TinyMCESpellChecker.SpellCheckerPlugin.GetConfigurationOptions()
at EPiServer.Editor.TinyMCE.PlugInAssembler.RegisterPlugin(TinyMCEPlugIn plugin)
at EPiServer.Editor.TinyMCE.PlugInAssembler.AddToolbarRow(IList`1 buttonNames)
at EPiServer.Editor.TinyMCE.PlugInAssembler.AddSettings(TinyMCESettings settings)
at EPiServer.Editor.TinyMCE.TinyMCEInitOptions..ctor(InitType initType, IDictionary`2 options, TinyMCESettings settings, IContent content)
at EPiServer.Editor.TinyMCE.TinyMCEInitOptions..ctor(InitType initType, TinyMCESettings settings, IContent content)
at EPiServer.Cms.Shell.UI.ObjectEditing.EditorDescriptors.XhtmlStringEditorDescriptor.ModifyMetadata(ExtendedMetadata metadata, IEnumerable`1 attributes)
at EPiServer.Shell.ObjectEditing.ExtensibleMetadataProvider.ApplyExtendersToMetadata(ExtendedMetadata metadata, IEnumerable`1 attributes, IEnumerable`1 metadataHandlers)
at EPiServer.Shell.ObjectEditing.ExtensibleMetadataProvider.GetMetadataForProperties(ExtendedMetadata parent, Object container, Type containerType, IMetadataProvider customProvider)
at EPiServer.Shell.ObjectEditing.ExtendedMetadata.get_Properties()
at EPiServer.Cms.Shell.UI.ObjectEditing.SettingsPanelMetadataExtender.ModifyMetadata(ExtendedMetadata metadata, IEnumerable`1 attributes)
at EPiServer.Shell.ObjectEditing.ExtensibleMetadataProvider.ApplyExtendersToMetadata(ExtendedMetadata metadata, IEnumerable`1 attributes, IEnumerable`1 metadataHandlers)
at EPiServer.Shell.ObjectEditing.ExtensibleMetadataProvider.CreateMetadata(IEnumerable`1 attributes, Type containerType, Func`1 modelAccessor, Type modelType, String propertyName)
at System.Web.Mvc.AssociatedMetadataProvider.GetMetadataForType(Func`1 modelAccessor, Type modelType)
at EPiServer.Shell.UI.Rest.MetadataStore.Get(String id, String modelAccessor)
at lambda_method(Closure , ControllerBase , Object[] )
at System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters)
at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass42.<BeginInvokeSynchronousActionMethod>b__41()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass37.<>c__DisplayClass39.<BeginInvokeActionMethodWithFilters>b__33()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass4f.<InvokeActionMethodFilterAsynchronously>b__49()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass4f.<InvokeActionMethodFilterAsynchronously>b__49()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass37.<BeginInvokeActionMethodWithFilters>b__36(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass25.<>c__DisplayClass2a.<BeginInvokeAction>b__20()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass25.<BeginInvokeAction>b__22(IAsyncResult asyncResult)
at EPiServer.Shell.Services.Rest.RestControllerBase.EndExecute(IAsyncResult asyncResult)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Isn't it quite clear? TinyMCESpellChecker module is missing/misconfigured?