- Full server backup is created instead of incremental one.
- "Full hostname" contains capital letter in Server Settings > Server Settings
With enabled debug logging for scheduled backup below information can be found in
C:\Program Files (x86)\Plesk\PMM\logs\backup-yyyy-mm-dd-time\backup.log:
CONFIG_TEXT: [2019-03-11 00:21:19.284|6068] Debug: Add virtual host content incrementally
[2019-03-11 00:21:21.832|6068] Debug: Unable to find last incremental backup, because of current backup has no version mark in the file name backup_user-data_1903110015.zip
If update is not possible for some reason you may try the following