OK then it is a qualified domain... so is that the way it is in the dir listing
i mean in your direct admin in the file manager do you access it by going to
/home/username/domains/domainname.nl/public_html/ow_cron/run.php
or
/home/username/public_html/domains/domainname.nl/public_html/ow_cron/run.php
or
/home/username/maindomain/subdomain/public_html/ow_cron/run.php
the addess you put in the cron will be the same way you access it via the file manager
do you have oxwall in both domain and sub domain? If so you may need to add the subdomain to your htaccess file as a custom folder so that oxwall knows its ok to use that folder.
I assume the subdomain site is working?