Can't comment on your specific situation, but a common cause of the "can't log in" on dias purchase page is because it's actually the purchase page for EU servers. To check, look for "us.runesofmagic.com". If it says only "runesofmagic.com", then that's your problem.
Have seen the US client's button for diamond buying link to the EU site, so it's quite common and totally RoM's fault for you to end up on the wrong page.