Just an observation which may be irrelevant. I am installing from scratch. During the initial curl... the report reads the following version numbers for Nextcloud (see below). I do not use NC on MIAB, but I find the variance in version numbers curious, as I am running the (supposedly) same script. I am probably doing the wrong thing by running curl... over and over again 
but here goes anyway:
Installing Nextcloud (contacts/calendar)...
Upgrading to Nextcloud version 23.0.8
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Then, after restoring a backup and running curl ... again and the report reads:
Upgrading Nextcloud --- backing up existing installation, configuration, and database to directory to /home/user-data/owncloud-backup/2022-10-28-02:15:17...
Upgrading to Nextcloud version 21.0.7
///
And running it again, it reads:
Upgrading to Nextcloud version **22.2.6**
///
Which fails with
Nextcloud or one of the apps require upgrade - only a limited number of commands are available
You may use your browser or the occ upgrade command to do the upgrade
///
And running curl... again:
First it reads:
Upgrading to Nextcloud version 22.2.6
Nextcloud or one of the apps require upgrade - only a limited number of commands are available
You may use your browser or the occ upgrade command to do the upgrade
///
but it does not fail and continues later with:
Upgrading to Nextcloud version 23.0.8
Nextcloud or one of the apps require upgrade - only a limited number of commands are available
You may use your browser or the occ upgrade command to do the upgrade
///
and finally fails with:
Nextcloud is in maintenance mode, hence the database isn't accessible.
Cannot perform any command except 'maintenance:mode --off'
Nextcloud is in maintenance mode
Maybe an upgrade is already in process. Please check the logfile (data/nextcloud.log). If you want to re-run the upgrade procedure, remove the "maintenance mode" from config.php and call this script again.
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So I removed maintenance mode and running curl... finally returns:
Nextcloud is already latest version
And the script ends without failing.