If I try registering with a disposable email address, I get this message:
The domain part of the email you've entered is not allowed. You will need to change your email address in order to continue. If you think this is done in error, please click
here to contact the administrator.
So, if you are seeing something else, the problem may be different. You can try using the link to contact "the administrator", but I'm not sure that will help...
I also tried registering as "new_test_user@icloud.com", and I just got the usual confirmation-email-sent message.
Could you have some malware in you computer? Are you in a country that could be considered as "suspicious" by the forum? Are you using some kind of anonymizing tool?