You have installed an agent on a machine connected to a NAS device. In What to back up, you select a network share residing on this NAS device. Backup completes with warnings similar to: "The process cannot access the file because it is being used by another process".
Acronis Backup 12.5 starting with Update 3.2 (build 11010+)
The guide describes how to set up and configure QNAP and Synology NAS devices for running Acronis backup agent using containers technology on top of these devices and how to protect data stored on these NAS devices.