Bug and problem reporting
The Javassist bug is reproducible and confirmed. While we are waiting for release 3.7.4 there is a possible workaround. In case of a live system it is advised to take the system off line first, and make sure that no members are logged on. Then temporarily disassociate the group from all member accounts, perform the group change, and re-associate the group with the member account(s). Finally, set the system on line again. Remember to always have a good backup.
Thanks a lot for the tip/work around.