I can't quite figure out where the issue is with this.

I have a client who creates a forum with some extended utf-8 characters in the title. And all was fine.

But then on the admin page which lists the forums, they put a # in to order the forum and submitted. When they did that, all forums disappeared. Turns out the Bo_Cat field in w3t_Boards gets wiped. Something must not be escaped right, and I think the file responsible is /admin/doforummanage.php - but I can't seem to find the issue here.

Hmm, Been trying to duplicate this on a 6.5 test board. So far, no luck. Does it do the same thing when they use non utf-8 characters?

No it only seems to kill it if there's utf-8 characters. I'll direct my client here and see if they can offer any other input.