Something I noticed earlier in your other thread (k!ng) is that when you enable this modification your URL for the registration page changes.
Your registration page with this modification turned off is the normal registration page
Code:
yourforum/register.php
With the modification enabled I remember it being something like this
Code:
yourforum/register.phpf1b0f2ba94a2db504cf33b29ab5314f5
Could that be causing this issue?