Hi Matthias,
I have little experience in this, but this seems related to a problem with the server failing to check the hash code in your page's postback v13wstat3, even if I have to admit that I don't understand german. This could happen when the server who receives the postback isn't the same that sent the page. This is a typical scenario in a web farm enabled installation. I wonder if you have a way to disable this, and check if the problem persists. As stated before, this is not a problem, since this check is provided only to prevent users from altering the v13wstat3 content with malicious intents, and it doesn't mean that the application is causing any unhandled exception.
Best regards,
Dario Rossa