That error "Warning: Cannot modify header information - headers already sent by (output started at /home/sth/mysite.com/rs-area/menu.php:3) in /home/sth/mysite.com/webassist/mysqli/authentication.php on line 206"
Usually means that you have a blank line of code preventing the redirect. In this case on line 3 of your menu.php file. If you attach a copy of that file I can be more confident of the exact reason.