Stuck in maintenance mode and incomplete backups

[mcnesium]

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?