[Zope] getUserObject Attrib Error

Hi All! Now and then there are strange things going on with my Zope 2.1.4/Linux Intel installation. Ok, here is what happens: http://www.myhost.de:8080 raises a Zope Error (Attribute Error, getUserObject) http://www.myhost.de:8080/manage requests authentication and I give as UID/pwd as the superuser's pwd. Again, strange Error-Messages. I repeat (non-deterministic number of times) this and suddenly: everything is ok. What can this be?? Regards, Ingo.