As the message says, Plex is performing maintenance tasks and is unavailable. For example, the PMS database schema frequently changes between versions. When this happens, Plex updates the db to the new schema and displays the 503 message as the system is unavailable when that is happening.
Wait for the message to clear. Do not interrupt the process. If you do, it will start over when Plex Media Server is restarted. Also, you risk corrupting the database.
The message is new, but the update process has not changed. Some users were interrupting the process and corrupting the database. Plex added the message to let users know what was happening and to wait until it is finished.
You should have a user account on the Synology with read-only or read-write permissions to the media folders. This can be any user account.
Use that account when adding the SMB shares to the Windows Server system where PMS is running.
If you change the password on the Synology user account you will need to change the password on the Windows Server system as well.
