Open your config.php which can be found in your forumroot/includes directory.
Put this at the top:
Code:
define('DISABLE_HOOKS', true);
above <?php
Save and try to re-register.
Should be no errors now.
Be advised this will disable all plugins. Once you know for sure its a plugin you can start turning them all off one by one till you find the culprit.
Remove the line I just said to add before you start turning off the products to find the problem.