I had also an issue with Error Code 503 Server unavailable 503 Error after cloning a sub-server .
In may case a configuration file in /etc/php/8.1/fpm/pool.d/ was missing
- The Apache Configration points to: SetHandler proxy:unix:/run/php/17392042948251.sock|fcgi://127.0.0.1
- /etc/php/8.1/fpm/pool.d/7392042948251.conf (was not created by the clone subserver task)
- The special file /run/php/7392042948251.sock was not created, because 392042948251.conf was missing
In the log files I found a hint that the socket was not available