Some upgrader wonkiness I guess. I've fixed Zarzal's. If you're brave you can do it too

Close your forums, cross your fingers, do a database backup if you're really scared

These two queries will remove everyone from group 4, and then add everyone back in. I'll need to automate this cleanup at some point. Zarzal's looks like they came from deleted users, but not sure.
delete from ubbt_USER_GROUPS where GROUP_ID='4'
insert into ubbt_USER_GROUPS (GROUP_ID,USER_ID) select '4', USER_ID from ubbt_USERS where USER_ID <> 1
Clicking on the Guest group it's normal for it to come up with zero results, since you can't edit user 1, the only person in that group.