Add a die statement to your recordset to see:
<?php
$colname_rsRegUser = "-1";
if (isset($_SESSION['MM_UserGroup'])) {
$colname_rsRegUser = (get_magic_quotes_gpc()) ? $_SESSION['MM_UserGroup'] : addslashes($_SESSION['MM_UserGroup']);
}
mysql_select_db($database_connHCHQ, $connHCHQ);
$query_rsRegUser = sprintf("SELECT * FROM users WHERE UserID = %s", GetSQLValueString($colname_rsRegUser, "int"));
$rsRegUser = mysql_query($query_rsRegUser, $connHCHQ) or die(mysql_error());
$row_rsRegUser = mysql_fetch_assoc($rsRegUser);
$totalRows_rsRegUser = mysql_num_rows($rsRegUser);
die($query_rsRegUser);
?>
My guess is a problem on the login page. Does anything associated with the MM_UserGroup session variable work?