As it states, the Plex server is performing maintenance tasks.
This generally means updating the database schema, which is common with PMS updates.
The correct course of action is: Wait for the maintenance tasks to finish.
The length of time to perform the tasks varies based on the capabilities of the system, the size of the database, and the amount of changes that need to happen.
Based on other posts in the forum, it can take 5 minutes, 30 minutes, or 2 1/2 hours.
Interrupting the task and/or falling back to another version is not recommended. Doing so (a) basically starts the migration over, and (b) can cause database corruption, since the database is open for writing when the migration is forcefully interrupted.
Once the migration is complete, the 503 message will go away and you can login to the Plex server.
I really doubt your database is 15.6TB, that might be the size of your library. but your database shouldnt be more than a couple GB at most. The log files should provide information as the database gets updated
I can help you with this but it will involve a tool I’m developing.
I would like to be involved because a 15TB database file is unthinkable in the Plex world. I’m sure Engineering would very much like to learn about it (since it can’t be shared)
I’ve also asked Engineering for guidance on handling this.
Something isn’t right and they will know best how to proceed.
( I have a good idea but would like their confirmation/blessing )