Yes, that.
Registration page is not broken, the API seems to be up, just today 3 persons registered ingame so it is not a TMW-side problem.
Remember it can take up to 15 minutes between completing the registration form and you being able to login in ManaPlus due to... Well, to be honest, I don't know the reason, but this happens, alright? Even if I were to manually create an account to you, there would be the same issue, it is on the C++ parts. (Blame o11c perhaps)
So, did you try logging in TMW 15 minutes after completing the form?
Did you block javascript or recaptcha? These can prevent registration as well.
This being said, here says "Medzero2022 registered Yesterday 19:20 BRT, account provisioning in progress" so I assume from our part, it went fine.
Remember that it is case-sensitive!! If you created account as "MEDZERO" and try to login as "medzero", it'll not find the account.