Thanks for your help. The issue is now solved.
The ATFolder "billeder" had a dangling reference to the
Normally it is possible to solve this by calling manage_delObjects on
the object with the dangling reference, but in this case
mange_delObjects threw an poskeyerror.
instead did the trick.
For more information about ZODB, see the ZODB Wiki:
ZODB-Dev mailing list - ZODB-Dev@zope.org