I happened to get this too, and the only solution to me was to remove the old config and start over. This means setting up Plex again. Shouldn't really be a problem, just a bit annoying.
As a side note: I happened to experience this because I decided to dedicate my GPU to a VM instead of giving Plex access to it. Once I removed my GPU from the Container settings I guess it broke something.
Here's what I did to fix it:
1. Stop the container if it is running.
2. Check the container settings to see where your config files are located. The default should be:
/mnt/user/appdata/binhex-plexpass
And make sure you take note of other fields you have changed, such as the media location. You might want to keep this identical when creating a new container.
3. use the console and execute:
mv /mnt/user/appdata/binhex-plexpass /mnt/user/appdata/binhex-plexpass-bkup
to "remove" the config folder. We keep a backup just in case.
4. Removed the docker container (no need to delete the image).
5. Add the container again and update your config-fields. I do not use a GPU, so my config looks like this:
6. Once the container is ready, go to the webui and append /manage to the url:
http://192.168.10.168:32400/manage
7. You can now add your media and Plex should be good to go.