Mediaindex not working any more after power loss

[timewarp]

After a power loss during a heavy thunderstorm my NAS F5-221 was recovering fine.
But since then the mediaindex and therefore also the multimedia app does not work any more.
If i set the checkbox to activate it and press ok, then a popup shows "configuration saved" and the checkbox is off again.
Checking the log: /var/log/mediadb/medidb.log i can see:

2023-08-06 22:16:30.859 CST [9685] FATAL: lock file "postmaster.pid" already exists
2023-08-06 22:16:30.859 CST [9685] HINT: Is another postmaster (PID 11527) running in data directory "/Volume1/@system/media/pgsql"?

Deleting the PID file does not help. Also not killing the mentioned process or doing both at the same time.

How can i get the index running again?