[Zope] Copy and Paste Problem

Hi all, I hope somebody knows an answer to my rather strange problem. It seems that in our Zope-Installation (2.74) we screwed up the Copy/Cut/Paste-Mechanism. We can not copy any objects in the ZMI at all. The result always is, that the same object ist pasted several times (Paste-Button stays at the bottom of the ZMI) or a copy error occurs. When we empty the caches/cookies etc. of the browser and restart it, we can copy one object, before the same behaviour starts again. Does anyone know what the problem could be? Thank you for your help. Greetings from Germany, Florian