One of your usergroups is setup to auto subscribe members to a forum, but you have not defined the forumid
I now have a fix coded in that halts the attempt to save a usergroup with those settings with 2.6 So just open up your usergroups and check each, or open and try to save each one and you'll get the error message
I'll add an error message into the auto_sub script for 2.7 for if there is not a defined forumid to tell you which usergroup to correct