Does the directory: /var/www/clients/client4/web21/home/web21 exist? If not, try to create it: chattr -i /var/www/clients/client4/web21 mkdir -p /var/www/clients/client4/web21/home/web21 chown web21:client4 /var/www/clients/client4/web21/home/web21 chattr +i /var/www/clients/client4/web21
That worked! The cron is now running successfully ... which actually makes me even more confused, since this directory now actually exists, it suddenly reads from the correct path (it reads from "/var/www/clients/client4/web21/web/" now, and not from "/var/www/clients/client4/web21/home/web21")... do you know why that is happening?
You can e.g. see the difference in the cron file in /etc/cron.d/, the shell is a jailkit shell and not /bin/sh when a cronjob is jailed. Create one and compare the files in cron.d/ to see the difference.