[xwiki-users] row was deleted by another transaction

Hello XWiki experts, with our XWiki 1.5 I encounter the following complaint at trying to save the document of a user, even after a restart of the servlet container:

org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [com.xpn.xwiki.objects.StringProperty#<?xml version="1.0" encoding="UTF-8"?> <eduLevelFine></eduLevelFine> ] at org .hibernate .persister .entity.AbstractEntityPersister.check(AbstractEntityPersister.java: 1765)

I am puzzled and have no idea where to go except into SQL, which is not really a safe thing. thanks for hints. paul