See this forum topic: http://ez.no/community/forum/developer/node_group_assignment_how_to_remove_correctly

The login handler is also affected, as the new user probably does not have the permission to remove nodes of it's own user object.

Probably related bugs:
http://ez.no/community/bugs/transaction_error_when_multiple_main_nodes
http://ez.no/community/bugs/crash_when_trying_to_copy_of_subtree_editors_to_a_fresh_user_group
http://ez.no/community/bugs/ldap_users_bug