<Response code="503" title="Maintenance" status="Plex Media Server is currently running database mig

Server Version#: current official docker image
Player Version#: nVidia Shield with all updates
I have been running Plex Server in a Docker container under Unraid for the last 3 years. Recently I updated to the latest version of Plex Media server & after the update, it was running but not working & when I attempted to connect to the web interface from a browser, I got the above error code. After trying some troubleshooting idea, I ended up uninstalling its Docker container. Today I try reinstalling the Docker container & upon connecting to the web interface, I get the same error message.
When I open the logs, I see the error messages below,
Starting Plex Media Server.
Error: Unable to set up server: std::exception (N2DB9ExceptionE)
Stopping Plex Media Server.
Starting Plex Media Server.
Please advise?

HTTP 503 == Maintenance message.

It is not an error.

It is PMS telling you it’s busy with the database and is temporarily unavailable until the database upgrades are complete.