I figured out th problem.
A program called "ApproveIt" added the following registry key: HKEY_CURRENT_USER\Software\Microsoft\Office\Excel\Addins\ADTAPILib.ApproveItAddin
and set the load behavior to 3. If I delete this key or change it to a value other than 3, everything works fine.
So ApproveIt is the problem...
Thanks for you help.