On Windows Server, a scheduled snapshot operation may fail due to mounted volumes being locked by the OS

A Windows Server operating system issue causes the operating system to intermittently lock mounted volumes. This can result in a failure in a scheduled snapshot operation, if the user specified mount points or mount paths for the snapshot volumes or manually mounted the snapshot volumes after a snapshot operation completed. If the operating system locks mounted volumes, when the scheduler tries to do the next scheduled operation, it fails with the error "volumes are in use". The error can be found in the .sts file corresponding to the schedule. (1205743)

Workaround: Check if any programs or processes are holding a lock on the storage groups and take the necessary steps to release the lock on the relevant volumes. Remove the mount for the volume before the next scheduled snapshot.