It's possible that something in the name you're trying to use isn't acceptable to the filter because of a combination of letters that may not be super obvious. Unfortunately, if the above is the case, then you'd have to use something else. It's not recommended to try and work around the filter, as that can end up leading to your name being Guardian'd, and if you already used your one-time name change before that, then you'd be stuck as Guardian for the foreseeable future.