Created Unassigned: AjaxFileUploadHandler - Invalid context key [27718]

Hi,

I dont know what to do - I always get a invalid key context exception when I use the AjaxFileUpload-Control.
The complete Exception looks like this:

Exception Details: System.Exception: Invalid context key

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:

[Exception: Invalid context key]
AjaxControlToolkit.AjaxFileUploadHandler.ProcessRequest(HttpContext context) +536 System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +625 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +270_

The URL that causes the problem looks good to me:
_.../AjaxFileUploadHandler.axd?contextKey={DA8BEDC8-B952-4d5d-8CC2-59FE922E2923}&fileId=A76A5AC2-8886-F19C-1252-EF068611EF05&fileName=picture.jpg&chunked=false&firstChunk=true&storeToAzure=false&acn=_

What could be the problem?
Thanks!