The bug is decribed here http://www.igniterealtime.org/community/thread/35411?tstart=0
The trunk is not handling user with an empty 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.
ContactList_empty_group.patch (9350 Bytes)