The most common login and registration problems are from either a bad DNS setting or a bad default gateway setting. First check that from the trixbox system you can ping registry.trixbox.org. If you can you might also have an older file that didn't update properly. To repair this, follow these instructions:
login to the box via ssh or the console
nano /var/www/html/maint/includes/functions/captureSoapIPURLList.php
use ctrl-k to erase all of the lines of this file
Paste in the following code:
<?php
$url = array('http://registry.trixbox.org');
?>
hit ctrl-o to save the changes and then ctrl-x to exit