Problem with Hyper-V backup (failed to open raw hard disc file)

We have an installation of Acronis Backup and Recovery 10 which has been problematic from day one. Recently, we started receiving the following error out of the blue and cannot backup any of the Hyper-V guest on the server. Any suggestions?

Error occurred while running the backup and recovery engine.
Additional info:
--------------------
Error code: 2
Module: 218
LineInfo: 600f7166ce397e0e
Fields: $module : C:\Program Files (x86)\Acronis\BackupAndRecovery\mms.exe
Message: Error occurred while running the backup and recovery engine.
--------------------
Error code: 94
Module: 83
LineInfo: 383ffdb092990da7
Fields: $module : C:\Program Files (x86)\Common Files\Acronis\BackupAndRecoveryAgent\BackupAndRecoveryService.exe
Message: Failed to open raw hard disk data file 'C:\Virtual Machines\Terminal Server\Terminal Server.vhd'.
--------------------
Error code: 64
Module: 83
LineInfo: 383ffdb092990d4f
Fields:
Message: An internal error.
--------------------
Error code: 33
Module: 7
LineInfo: e24f93d0b8d8c9c3
Fields:
Message: Unable to create volume snapshot
--------------------
Error code: 50242
Module: 16
LineInfo: 14181c22ef45b04e
Fields:
Message: Failed to start creating the volume snapshot.
--------------------
Error code: 9
Module: 0
LineInfo: 2aacb7b2ab852ac
Fields: code : 2147754760
Message: Unknown status.
--------------------
Error code: 65520
Module: 0
LineInfo: bd28fdbd64edb8bc
Fields: code : 2147754760
Message: Unknown Win32 error
--------------------