Backup does not start app.zip not created

Hello,

I configured the nextcloud backup app with a local and a external storage and tried to manually create the first backup. As it seems to be the case for some other users the message " The creation of a restore point has been requested and will be started soon." keep showing like forever but nothing is created. So today I tried to start the backup via terminal with

sudo docker exec --user www-data ubuntu_app_1 php /var/www/html/occ backup:point:create

And I got the following output:

maintenance mode: on
initialization of the AppData
initialization of the RestoringPoint: 20220415181947-full-qp9kL6gNVE1NMzx based on NC23.0.3.2
initialization of the storage
preparation of the data to be stored in the restoring point
preparation of internal data
maintenance mode: off

In ChunkService.php line 722:

Could not generate app.zip

backup:point:create [--output [OUTPUT]] [--generate-log] [--differential] [--] []

When I go to the local folder where the backup should be created I see a folder with the above mentioned name of the RestoringPoint but it is empty. I can create files within from the webinterface of nextcloud, so I it seems to me the rights should not be an issue, but asI am not very familiar with how the Backup App works I still would like to mention that I restricted the access rights for both of the storages where the backup is to be stored to only be accesible for nextcloud admins from within the nextcloud external storage app.

Is there anything I can do to help identifying the exact problem?

Start with ChunkService.php around line 722 and reproduce the failure using the reported docker exec occ backup:point:create command. Inspect the empty restoring-point folder and the command output; the work is done when the backup completes and app.zip is created in the configured local storage.