account root should be owned by root
When you create a shell account it depends on the selection of jailkit if the dir such as /var/www/clients/client1/web42
get's chowned to be owned by root or the user being created.
Jailkit depends on it being owned by root. So when you first create a shelluser with jailkit enabled, and then one without it the first user becomes inaccessible.
jk_chrootsh[XXX]: abort, /var/www/clients/client1/web42 is not a safe jail, check ownership and permissions.
See shelluser_base_plugin.inc.php and create_jailkit_chroot.sh
I see no reason why the user should own that dir himself, we make it read only with chattr anyway.