Problem solved, but wierd issues.
When you set the forum default language, that doesn't mean that the language value will be set for all users in the DB. It's a global setting and not something written to the DB when a new user is created. I didn't change anything for default groups that a user belongs to. I don't know how that was set to nothing. All users before the upgrade went to group 4 (users), after the upgrade they went to group 3 (moderators). That was because somehow no default group was set.
I maintain a secondary domain running out of the same database for reasons that aren't needed here. It works as long as the config file is set correctly. Anyway, that site worked. So, it wasn't the DB. In the end there was some sort of weird interaction with the content islands/gallery and the forums we had messed up. Since I solved the problem by copying some files from the duplicate installation, I don't have the exact solution.
Suffice it to say, that this ended up being a complex set of interactions. I really appreciate those that worked to help out. This issue can be closed out.