No element in container

[{Reint Hartlieb}]

Hi,
 
In a workflow i have a workitem that is appointed to a user by a rule. It does appear in the mailbox of the user, but can not be executed. Sap responds "No element in container" and leaving it in the inbox. 
I can't find faults in the containerbindings. (The containerinstance of the workflow is repeated in the containerinstance of the workitem.)
 
What is the nature of this error and how can i correct it?
 
Greatings, Reint
 
 
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