I just registered and could indeed cuplicate the effect. There is something wrong in your login page. Here us the URL it is trying to go to after a login:
(sid deleted as irrelevant)
Have a look at your login.php file (forum root) - you might just re-upload a clean copy of both config.php and login.php and make sure it's being uploaded as ASCII/text and not binary. See if that helps.
On my way out soon but let me know if that doesn't fix it and I'll look further.
Which version of phpBB is it, by the way?