I found the problem! There was a short PHP tag on addlink.php line 407 so servers which do not allow short PHP tags threw an error. I changed it to a full tag now in the original download so you can either download linkman again and upload the new addlink.php file or change line 407 yourself from
to
Sorry for the inconvenience, most servers have short tags enabled (they are enabled by default in PHP) and I didn't even notice it.