I suddenly have this really annoying problem. It showed up on its own last week. At least, that's when I heard of it. People could not sign up as any link to wp-signup.php redirected back to the main page.
I got a fresh copy of the file, pasted it in, problem solved.
This week, I've made edits to that file to moderate spam signups, as *of course* spam blogs were still able to sign up with or without the redirect issues. The edits I made were tested on another account on the same damn server. It worked there. :-/
The error logs are empty, htaccess file is the same as everyone else's. Any ideas?