Stuck in maintenance mode and incomplete backups

Running the Backup app for about 11 months on a NextBox, I keep seeing the following issue:

• ~2300 directories today with about 10MB size each exist on the remote restoring point (another Nextcloud instance, mounted as an external storage)

• not a single backup restoring point has been completed

• the entire instance keeps stuck in maintenance mode randomly, and is only available at a fraction of time. I have monitored the HTTP status codes of the instance for 9 months and visualized them. They are only 503 (maintenance) or 302 (available):

Here is the live chart with zoom functionality

This is the config of the Backup app:

Both Nextcloud instances (the NextBox one and the external storage one) are being updated automatically and now at NC25.
The external storage machine is connected to the internet through a Vodafone 1000 cable network connection. I don't think a poor connection is causing this.

So what is causing this?

No files, tests, or entry points are identified. Reproduce the described Backup app setup with the NextBox instance, external Nextcloud storage, and maintenance mode enabled, then inspect the backup and restore-point behavior. Done means identifying why backups remain incomplete and the instance stays in maintenance mode, with a verified fix or clear diagnostic result.