The path web/sendmails.php is wrong. You have to use the absolute path to the script starting with a /
Jul 23 19:46:01 prl-ofc-s01 jk_chrootsh: path /var/www/clients/client1/web14/dev/ is not owned by user 0
You seem to have added a folder dev in /var/www/clients/client1/web14 which is not owned by root and this prevents the creation of the cronjob jail. Remove this folder or chown it to root.