Aye! That did the trick. The line reads now
// -- Bugfix for Xitami, courtesy V@no)
// header("Location: ".$site_sess->url($url, "&"));
header(((@preg_match('/Microsoft|WebSTAR|Xitami/', getenv('SERVER_SOFTWARE'))) ? 'Refresh: 0; URL=' : 'Location: ').$site_sess->url($url, "&"));
categories.php 1; details.php 2; download.php 4; global.php 1; login.php 4; logout.php 2; postcards.php 3; member.php 7 instances to correct, if anybody else should experience the same problem.
Thanks V@no for your tireless support.