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Cannot access test server is indeed the message you get when you are banned. Normally this appears when you are permanently banned. Most common reasons for a perm ban: getting caught too often by the autoban, found hacking in a report, chargeback. Game bans and forum bans are usually separated, so being able to log in on the forum means nothing about the state of your account.
In case of chargeback or if you don't know why you were banned: take it up with support.
They will tell you the reasons for your ban and if there is anything you can do to reclaim your account (only in case of a chargeback).
If you were hacking, don't even bother messaging support.