Please reinstall / update the Plex Debian package to make sure everything in /usr/lib is really as it should be and please never touch this directory again I suppose you make use of the package repository, so it’s a sudo apt-get install --reinstall plexmediaserver or just sudo apt update && sudo apt upgrade as you are using an old version.
Now, as you still specify pi as group and user in the override.conf file, PMS cannot write into your Preferences.xml file, because it’s owned by plex again, right? So you have two options:
(1) use the default plex:plex setting: remove the override.conf and fix file permissions of your media to allow access to the Linux user plex
(2) not recommended: keep your override.conf, and chown everything in /var/lib/plexmediaserver again to pi:pi.
Always remember to restart PMS with sudo systemctl restart plexmediaserver after changing the file system structure / permissions.
The first thing I did was exactly that (with the exception of the reinstall of Plex).
I changed the override.conf to user pi:pi, then daemon-reload, then chown -R /var/lib/plexmediaserver to pi:pi and started the server.
Server settings are unavailable.
Change override to plex:plex and chown to plex:plex and everything is working. So I don’t understand your argument that Preferences.xml was not writable. Where is this located? In /var/lib/plexmediaserver?
By the way, I’m using my library in the default location. Override just changes the user.
I forgot one important fact: PMS by default looks for its settings in $HOME/Library/Application Support of the user it is running at. /var/lib/plexmediaserver/ is the default home of user plex. Whenever you change the user, e.g. to pi in the override.conf, PMS does not use /var/lib/plexmediaserver/ anymore but e.g. /home/pi/Library/Application Support. That’s why there is the optional Environment setting in the override.conf file.
In result: if you want PMS to run as pi with the server settings still in /var/lib/plexmediaserver you also have to set Environment="PLEX_MEDIA_SERVER_APPLICATION_SUPPORT_DIR=/var/lib/plexmediaserver/Library/Application Support".
Preferences.xml is located by default in $HOME/Library/Application Support/Plex Media Server.
Whenever the claiming does not work as expected, it’s usually because of missing write permissions to Preferences.xml.
Preferences.xml contains the ID of your server and other important settings. Removing this file allows you to reset your server, but you have to reconfigure e.g. shares.
Do you know how to copy the logs directly from your RPi without the web UI? Please post /var/lib/plexmediaserver/Library/Application Support/Plex Media Server/Logs/Plex Media Server.log and take a look at its creation time, to make sure PMS is really currently using this directory: ls -ahl "/var/lib/plexmediaserver/Library/Application Support/Plex Media Server/Logs/Plex Media Server.log".
Thank you very much for your support.
Since we last talked, I have installed an arm64 in my RPI 4B+ with Ubuntu. Yeah, it has it’s benefits but for sure has its downsides. Anyway, I have executed the same steps I did in Raspberry PI OS, like moving the library and altering the systemd entry and everything worked flawlessly. So I guess it was an OS issue, not a Plex issue.
I have now abandoned Raspberry PI OS and rebuilt my library from scratch, with minor adjustments needed. It’s good that I did not have to reinvite people, I just shared the new libraries with them.
The only thing I would love your advice on is regarding the /usr/lib/plexmediaserver folder. See, /usr/lib is under an SD card and I would prefer to have anything IO intensive in my HDD. What’s lying there and is it IO intensive? Would I get performance improvements moving that folder to my HDD? Should I use symbolic links or do you have a magic attribute to use in the override file?
FYI. I did not like Ubuntu so much so I decided to give it a try on Raspberry PI IO arm64 and did the same I did back in the old Raspbian and now it worked perfectly.
Maybe there was something wrong with my machine-id something like that.
I suppose you make use of the package repository, so it’s a 
