As you can see, all configuration files AND log - file - locations from Plesk are defined there and you are now able to have a look at the "Backup Manager" - logs to investigate your issue.
Sometimes, it is as well a good idea to change the log - level, to get more informations in psa - log - files:
If you have questions to your investigation, pls. consider to post as well depending log - file entries, so that people willing to help you, don't have to guess, what's happening on your server.