Unable to complete backup over SFTP

NC 24.0.4
Backup app 113

Backup fails after writing some metadata to external storage.
Fails when the backup is configured to write over sftp

Any idea?

/home/user/cloud $ php occ backup:point:create
> maintenance mode: on
> initialization of the AppData
> initialization of the RestoringPoint: 20220905085730-full-cwsnFq9muHLAMoe based on NC24.0.4.1
> initialization of the storage
> preparation of the data to be stored in the restoring point
> preparation of internal data
> maintenance mode: off
An unhandled exception has been thrown:
TypeError: fwrite(): Argument #1 ($stream) must be of type resource, bool given in /home/user/cloud/lib/private/Files/Storage/Common.php:209
Stack trace:
#0 /home/user/cloud/lib/private/Files/Storage/Common.php(209): fwrite(false, '')
#1 /home/user/cloud/lib/private/Files/Storage/Wrapper/Wrapper.php(258): OC\Files\Storage\Common->file_put_contents('backup/20220905...', '')
#2 /home/user/cloud/lib/private/Files/Storage/Wrapper/PermissionsMask.php(112): OC\Files\Storage\Wrapper\Wrapper->file_put_contents('backup/20220905...', '')
#3 /home/user/cloud/lib/private/Files/Storage/Wrapper/Wrapper.php(258): OC\Files\Storage\Wrapper\PermissionsMask->file_put_contents('backup/20220905...', '')
#4 /home/user/cloud/lib/private/Files/Storage/Wrapper/Availability.php(274): OC\Files\Storage\Wrapper\Wrapper->file_put_contents('backup/20220905...', '')
[...]

Reproduce the failure with php occ backup:point:create using an SFTP destination, then start at lib/private/Files/Storage/Common.php:209 and follow the storage wrapper frames shown in the trace. Done means identifying and resolving the failure so the backup completes over SFTP.