Acronis Backup Cloud: Virtual Appliance backup fails with "Cannot find GXT item type 'mms::disk::disk'."

Symptoms

1. You deploy Acronis Virtual Appliance to a ESXi host and it appears in the Devices list in backup console.

2. You apply a backup plan to Acronis Virtual Appliance. 

3. Backup of Virtual Appliance fails with a 'Failed to execute the command. Resolving items to back up' error containing these messages: