I'm pretty sure it's bug, but if someone confirms it then it can be moved.
Go to Membergroups > Select group (post based) > change name > Save:
The database value you're trying to insert does not exist: subs_description
or (board Moderator group):
The database value you're trying to insert does not exist: subs_online_color
/edit:
forgot to add error log:
/index.php?action=admin;area=membergroups;sa=edit;group=3
File: /forum/sources/subs/Membergroups.subs.php
Line: 1374
and I have to add that it worked fine on RC2.