I'm not sure what the issue is, but I see that file has:
<?php require_once("webassist/email/WA_Email_temp.php"); ?>
When I test with:
<?php require_once("webassist/email/WA_Email.php"); ?>
It appears to work. I'm not sure what is different about that file, or why you have changed it, but it appears to be causing this issue.