[RDX1]

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<form action="register.php" name="register" method="post" onsubmit="return verify_passwords(password, passwordconfirm);">

Change to:

<form action="path/to/your/forum/register.php" name="register" method="post" onsubmit="return verify_passwords(password, passwordconfirm);">

Change path/to/your/forum/ to http://www.yoursite.com/forum/path/register.php

I'm going to change it so it includes yoru site path automatically for no more confusion.

Edit: Try the new code, not idea why I didn't think of this before. I changed the paths to $vboptions[bburl]/ so it automatically checks the forum path, no editing required.