Forms authentication failed "The ticket supplied was invalid" for users using a proxy

Since we updated our website to .NET 4.0, users using a proxy-server can't login in our webapplication anymore.
In the event-viewer on the server this error is thrown:
Forms authentication failed for the request. Reason: The ticket supplied was invalid.This problem only exists for users with a proxy, all other users can login normally.
We are not using a web garden or multiserver (load balance) environment, just a webserver and a databaseserver.
Please help







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