What other plugins do you have??
Board Copyright (1.0)
Add Spiders (0.1)
ChangUonDyU - Advanced Statistics (1.0)
Game Section (1.2 RC3)
Gravatar (1)
Guests and lurkers combined message (1.0)
IMDB Tag (1.0)
MyMood (1.0 Beta)
Private Messages Admin (2.4.1)
reCAPTCHA Plugin (1.0)
Register Box (1.2)
SEO Links! (1.1 Beta 1)
Signature image limit (1.1)
Disable reCAPTCHA, that's the only one that sounds like it affects the registration page.