Unable to create a backup in Plesk: Failed to exec backup_restore_helper: Exit c

[Administrator]

Symptoms
Backups created in Tools & Settings > Backup Manager display a warning:

Warning: Extension "Git"
Unable to back up extension. Error: Failed to exec backup_restore_helper: Exit code: 3: System user unavailable

Git extension version 2.3.0-383 is installed on the server
Cause
A Git extension bug, which has already been resolved in the latest update. The warning does not affect the backup integrity and is only cosmetic.
Resolution
Update the Git extension to the latest version 2.3.1-386