Ok, so time for the solution...
The PHP "max_input_vars" on a good host will be at least the default value (1000), anything less is not recommended. Where this setting at its default is a pretty good option, when you have a system like the member management system in UBB.threads which doesn't paginate the output, you have X amount of variables per entry (we'll call it 10 for the sake of not wanting to go and find the number again) and you have a lot of pending users, you'll hit this limit at 100 users (default max_input_vars setting); in this particular instance there are thousands unapproved users, hence why the form wouldn't accept the input on any user (because it was sending the payload of the entire page).
We set the "max_input_vars" to a huge number so that the listing could be processed, and will set it lower once it's done. If you do not have access to your PHP configuration you'll need to delete pending users that you would otherwise deny, and set the "USER_IS_APPROVED" column to "yes" in the "ubbt_USERS" table for each user you wish to approve manually.
