wwe run quite a number of vBulletin forums, but only one of them has the problem I'm about to describe, and it has followed us through four upgrades, so it can't be an issue with the templates. Users are unable to delete their subscriptions. They select them, select delete, hit the button and nothing happens. The page merely refreshes. I can delete them from the admin CP with no problem.
We can only assume that the issue is in the database, since as I said it has followed us since version 3.0. We suspect there may be some usernames with illegal characters. How can I find a listing of characters which are natively not permitted in vBulletin usernames? I know how to set my own, but we currently have none set.