Can't create new forum topic, as Forums menu is empty

OK, I know theoretically no one's supposed to be using this module any more until the security issues are resolved, but I have one site that depends on it, and it's a completely private site where there are a small number of known users, and anonymous users can't access any part of it whatsoever, so I think it's less vulnerable than most. But with that out of the way, here is the issue I'm having trouble with now:

I recently upgraded the site, at long last, from Drupal 5 to 6 (I'd delayed on that for a long time because it's a complex site and I knew it would be challenging to upgrade - and as it turns out, challenging was an understatement). And a serious problem has turned up in the wake of the upgrade: new forum topics can't be created in the OG forums, which are pretty much the lifeblood of the site. On the create topic form, there's a required field called "Forums", and it's blank - it doesn't show the forum that exists within each group. Not even if you access it from directly within that forum. And the form can't be submitted without choosing a forum, which you can't do from a drop-down menu that's empty.

I can see the forums if I look under admin/content/forum and also if I click on "group forums" under the group menu in the sidebar, and replies can be posted to existing forum topics, but for some reason the create topic form can't find the forums, so no new topics can be created.

Does anyone have any idea what might be going on here, and how I can fix it? This is really important, as the client this site belongs to depends on those forums on a daily basis (it's for a training program, and the forums are used for private communication between students and their teachers and advisors)