on the login page, change the form tag:
<form action="<?php echo ($_SERVER["PHP_SELF"]);?>" method="post" id="WAATKLogInForm">
to:
<form action="<?php echo $_SERVER["PHP_SELF"]; ?><?php echo (isset($_SERVER["QUERY_STRING"]) && $_SERVER["QUERY_STRING"] != "")?"?".$_SERVER["QUERY_STRING"]:""; ?>" method="post" id="WAATKLogInForm">