Cannot begin transaction. database is locked

[EspenLund]

Yup, that option was selected and still is.
It is back up and running after deleted Preferences.xml as per this thread:

PMS Ubuntu 18.04.02 Not fetching meta-data and server settings unavailable

With PMS stopped. Delete the Preferences.xml file in /var/lib/plexmediaserver/Library/Application Support/Plex Media Server Now start it. It will start up as “Unclaimed” and generate a new set of ID codes. Any media scanned in the DB is still retained. As you walk through the wizard, do not create new sections. All existing library sections survive this soft reset.

Will report back if it happens again.