At the moment, it's the matter of checking the log for a rule hit on your site for that specific page. From there, it could say what rule was triggered. That rule then can be disabled if it's more of a false-positive than an actual security risk.
In addition to what Angus said, we can't really help unless it's found to be legit. If we just help and it's an illegal copy then it would be like helping someone set up an illegal copy of forum software, which is a big no-no.
It's likely saying that your IP doesn't match the one used during the registration process.
Funny thing is that I can log in on with different IPs (like via mobile network).
Causes that I know of is that it could either be the .htaccess file containing something that would be considered non-exsistant (or invalid) or a bad header being set in a PHP script.
What plugin would this be?
Premium is on completely different servers and so are likely to not have these kinds of issues. (Or at least shouldn't.) (Free = many users = potential instability = more restrictions)
If you were using Filezilla and didn't turn off the PC then the edited files can still be around in a Filezilla temp folder on your PC, though you would have to redo the directories.
Note that if you're able to and if they don't have a backup.