Error on Thanks Page

I am using aMember Pro with the twocheckout_r plugin. On passback to the thanks page, users always error out with:

Unable to validate that you have paid, please contact the webmaster

I've poked around in thanks.php, and have been able to narrow it down to the code block:

if ($paysys_id){ //should be passed from url or plugins/payment/../thanks.php
//process plugin work if payment system pass info to thanks page
if (($error = plugin_validate_thanks($paysys_id, $vars))
|| ($error = plugin_process_thanks($paysys_id, $vars))
){
$t->assign('error', $error);
$t->display('thanks_error.html');
exit();
}
}

The credit card is being charged, so twocheckout is definately validating it correctly.

Please help!