given assembly name or codebase was invalid

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The given assembly name or codebase was invalid. (Exception from HRESULT: 0x80131047)

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.IO.FileLoadException: The given assembly name or codebase was invalid. (Exception from HRESULT: 0x80131047)

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[FileLoadException: The given assembly name or codebase was invalid. (Exception from HRESULT: 0x80131047)]
   System.RuntimeTypeHandle.GetTypeByName(String name, Boolean throwOnError, Boolean ignoreCase, Boolean reflectionOnly, StackCrawlMarkHandle stackMark, Boolean loadTypeFromPartialName, ObjectHandleOnStack type) +0
   System.RuntimeTypeHandle.GetTypeByName(String name, Boolean throwOnError, Boolean ignoreCase, Boolean reflectionOnly, StackCrawlMark& stackMark, Boolean loadTypeFromPartialName) +314
   System.Type.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase) +95
   System.Web.Compilation.BuildManager.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase) +124
   System.Web.Handlers.AssemblyResourceLoader.EnsureHandlerExistenceChecked() +11461706
   System.Web.Handlers.AssemblyResourceLoader.IsValidWebResourceRequest(HttpContext context) +16
   System.Web.Security.FormsAuthenticationModule.OnEnter(Object source, EventArgs eventArgs) +200
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +80
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +270

given assembly name or codebase was invalid

After upgrading to CMS 6.0 from CMS 5.2 i am getting this errormessage:

The given assembly name or codebase was invalid. (Exception from HRESULT: 0x80131047)