It was not possible to complete the request due to file

I'm not a PHP expert, just a little newbie, but if you state that you turned safemode off, that means that previously it was enabled, and from what I've understood about that "feature" whatever permissions that were set during that time will remain making it impossible to write to a folder that was created by a script.

I would try to delete the /ckfinder/userfiles/ folder, make sure that safemode is disabled and then assign the write permissions. Only after that, try to use CKFinder with some settings like these:

$baseUrl = '/ckfinder/userfiles/';
$baseDir = '/domains/domainname.nl/public_html/ckfinder/userfiles/';

but other people with more experience might give you some better advice

$baseUrl = 'http://www.domainname.nl/ckeditor/ckfinder/userfiles/';
$baseDir = '/userfiles/';

Check server logs, there should be more information about the source of "Internal Server Error".