The forum is the only place I can access successfully for the time being.
Tough luck, buddy - this forum is the most unhelpful place for you to find support. You're not allowed to discuss bans here. You what now? You're not banned, your account is just blocked? Don't you worry - there's an unwritten rule for that: "If you can't access your account, you count as banned, therefore you're not allowed to say a word on this forum." From the secret Useless Tera Forum Moderator Manual studentbook.
Joke aside, it is indeed pointless to seek help here. Moderators only care about protecting scammers and hackers. They don't care about players problems - they tell you to contact Support even though you may have written you already have. lol In this case they tell you to do it again because they can't help you and because they can't help you they assume everyone is as incompetent and uninterested in helping as they are. There is a support section for players to seek help from others players. But noooooooooooo, as far as moderators are concerned, they are the only players on this forum.
And instead of trying to help by letting others help, they tell you to go to third-party websites where you get exposed to malicious content etc. Just pathetic...
I successfully entered it in on the site, but when I tried to log into the game using the launcher, it told me my account was blocked. I tried to access the site afterwards, and it said that "An error occurred. try again later".
How long between your last website visit and your launcher login attempt? Have you forgotten some VPN add-on active, maybe changed Internet connections, or got your IP changed due to having a dynamic IP which sometimes is known to cause issues with account access? It is indeed possible someone tried to log into your account from a different location - it could've been you but system couldn't have known that. Support will be able to tell you what triggered the block.
What Racial is trying to say about changing passwords is that you need to register on the Support website separately if you, in 5 years, have never done it before. You can also just use your Steam profile to log into that system but you won't be able to see your past tickets if you had any at all. You need to link your regular Support account to your Steam profile before you can see your ticket history - just like you link your regular game account to your Steam profile if you want to log in via Steam.
As for the 'an unexpected error' error - there are a few reasons for that, including having your account blocked, unless you meanwhile did some hosts file changes or blocked Tera in any other way (Win Defender, some firewall-capable software etc). It's weird to hear that a browser change could help with that - if this was the case, then something in one's browser add-ons or settings was causing the issue. In any case, don't attempt to log into your account for 48 hours or until Support have told you they've unflagged your account as blocked, whichever comes first. Often they tell you it's OK but in fact it's not, so you have to message them back. It is usually then when they finally do it properly with or without apologising about it. Why not try logging in? Because doing that supposedly resets the block countdown.
Have you tried resetting the block by using a different Internet connection, such as your mobile phone connection? This is known to have dealt with this problem in the past. Same goes for the old account management but it's just luck based whether it works or not. Instead of making it easier for people to regain access to their often TC paying accounts, they keep making things worse, wasting players' time and money.
Good luck! 