In another server instance hit by https://forum.cloudron.io/topic/1382/error-for-multiple-apps/12, recreating docker containers did not help. I even disabled apparmor altogether using sudo systemctl stop apparmor and sudo systemctl disable apparmor. We tried server restart as well and that didn't help either.
After disabling apparmor, I saw that mysqld would just quit without showing any error message or log output 😞 As a final thing, I just ended up moving platformdata/mysql to mysql-old and recreated containers again. That made mysql come up and then we restored all the apps.
Still a bit crazy/worrying that we hit this bug atleast twice now and there is no clear resolution.