I apologize for the delay - things have been a bit busy!
I just tested this and was not able to recreate the error.
I created a signup-xtra.php with some simple code:
<?php// example?>
...and set the Custom HTML "Signup Sucess" page to forward me to listmailpro.com
and I was successfully subscribed and forwarded to the page.
I also tried setting signup-xtra.php to:
<?phpecho "this is a test";exit;?>
I saw the output and then the script exited before forwarding (as expected). It seems there must be some unexpected output from your signup-xtra.php script.
If you want to email me (dean@...) with a copy of the script you are trying to use and
submit your info so I can do some live testing on your installation maybe I can shed some light on this.
Regards