The bug is decribed here http://www.igniterealtime.org/community/thread/35411?tstart=0
The trunk is not handling user with an empty <NULL> group entry well. These kind of entries are created by an error on the OF server. My personal account in our production system has 4 out of 150 contacts with empty groups. Apparently these were created by users that move out of a shared group. Other clients handle the issue well.
The attached patch was tested against the SVN code base 10989 and works well.