Backup cannot be created with the following error:
Not enough free disk space to backup selected objects. At least 2140 Mbytes free disk space is required.
Software issue with id #PPA-2291. Plesk Automation (PA) checks available space on the Management node instead of Backup Service node. The issue was fixed in Plesk Automation 11.5 Update #10.
Install latest PA updates according to the article.