When you want to register a new account, you enter the main page (https://hydrogenaud.io) of this forum and you click on the "register" link (Please login or register) on the top left, it will take you to this link, which is just again the main page of the forum:
https://hydrogenaud.io/index.php?action=register%20rel=
the "%20rel=" suffix is damaging the link, and when removed, the user can get to the real register link:
https://hydrogenaud.io/index.php?action=register
But not everyone knows how to do that.
I logged out and clicked the registration link on the main page. There is no %20rel= in the resulting link.
E: Wait a second. I clicked the big Register button in the toolbar. I did not check the "Log in or register" links.
E2: I have fixed this. Someone made a terrible mistake in the English language template file. There are probably other mistakes like this, but I'll look for them later.