I am also having this same problem, I also have an issue when I click on the "catalog" option in the menu, it cannot find the "images" directory. Jan, your suggestion regarding the host setting "safe_mode" sounds very logical, as I had tested my implementation 100% on my local testing server, only to find these aren't working since uploading onto the live server.
I removed the @ in line 112 of admin/modules.php and it now shows a more detailed error
Warning: dir(http://www.<domain name>/catalog/includes/modules/payment/) [function.dir]: failed to open dir: not implemented in /catalog/admin/modules.php on line 112
My live server is administered by a third party company, what should I be asking them to change regarding the "safe_mode" setting? Sorry for my vague question, I don't know much about Unix/Linux web servers.