I'm sorry to hear you are thinking of leaving the site. I think the problem with the "alpha numeric" thing is that there must be a character in your username that N54 doesn't like eg. spaces, ?, <, >, ? etc. I think _ is ok but it is probably best to make sure your username is all one word with no spaces.
If you are still having problems, you can e-mail me directly on email@example.com with your chosen username, pasword and chosen e-mail address and I can sign you up from my end. I will then delete your details.