Backup Failing with VSS/RPC Errors

We have a Backup Exec job which is failing with the following errors:
"The directory or file was not found, or could not be accessed."
"Directory not found. Cannot backup directory ... and its subdirectories."
"An unexpected error occurred when cleaning up snapshot volumes. Confirm that all snapped volumes are correctly resynchronized with the original volumes."

After some digging and getting nowhere, I decided to try and run a job using the local Windows Server Backup on the target server (2008 R2). I selected the Full VSS option and ran it. The job completed and backed up all the files I had selected but the status was reported as failed.

Looking in the Event Viewer, I found two error entries for the backup:

-   System  
-   Provider
            [ Name]     VSS
-   EventID     8193
            [ Qualifiers]   0
    Level   2
    Task    0
    Keywords    0x80000000000000
-   TimeCreated
            [ SystemTime]   2019-12-23T10:29:32.000000000Z
    EventRecordID   3962947
    Channel     Application
    Computer    xxxx.xxxx.com
    Security
-   EventData
            IVssAsync::QueryStatus
            0x800706ba, The RPC server is unavailable.
            Operation: BackupComplete Event Executing Asynchronous Operation Context: Current State: BackupComplete
2D20436F64653A2042554543584D4C4330303030353934302D2043616C6C3A2042554543584D4C4330303030353833392D205049443A202030303030393736342D205449443A202030303031303234342D20434D443A202022433A5C57696E646F77735C73797374656D33325C7762656E67696E652E657865222020202020202D20557365723A204E616D653A204E5420415554484F524954595C53595354454D2C205349443A532D312D352D313820

and

-   System
-   Provider
            [ Name]     Microsoft-Windows-Backup
            [ Guid]     {1DB28F2E-8F80-4027-8C5A-A11F7F10F62D}
    EventID     517
    Version     0
    Level   2
    Task    0
    Opcode  0
    Keywords    0x8000000000000000
-   TimeCreated
            [ SystemTime]   2019-12-23T10:29:37.528986500Z
    EventRecordID   3962948
    Correlation
-   Execution
            [ ProcessID]    9764
            [ ThreadID]     7852
    Channel     Application
    Computer    xxxx.xxxx.com
-   Security
            [ UserID]   S-1-5-18
-   EventData
        BackupTime  2019-12-23T09:39:09.669986500Z
        ErrorCode   2155348143
        ErrorMessage    %%2155348143

This states quite clearly that the RPC Server is unavailable. So I have checked the Services to see if something had stopped.

RPC Server - Started - Automatic
RPC Endpoint Mapper - Started - Automatic
Virtal Disk - Manual
Volume Shadow Copy - Manual
Windows Remote Management - Started - Automatic (Delayed)

I've also checked the VSS Writers

Writer name: 'Task Scheduler Writer'
   Writer Id: {d61d61c8-d73a-4eee-8cdd-f6f9786b7124}
   Writer Instance Id: {1bddd48e-5052-49db-9b07-b96f96727e6b}
   State: [1] Stable
   Last error: No error

Writer name: 'VSS Metadata Store Writer'
   Writer Id: {75dfb225-e2e4-4d39-9ac9-ffaff65ddf06}
   Writer Instance Id: {088e7a7d-09a8-4cc6-a609-ad90e75ddc93}
   State: [1] Stable
   Last error: No error

Writer name: 'Performance Counters Writer'
   Writer Id: {0bada1de-01a9-4625-8278-69e735f39dd2}
   Writer Instance Id: {f0086dda-9efc-47c5-8eb6-a944c3d09381}
   State: [1] Stable
   Last error: No error

Writer name: 'System Writer'
   Writer Id: {e8132975-6f93-4464-a53e-1050253ae220}
   Writer Instance Id: {4be149e4-3213-47d3-94ad-c5fd96b3816e}
   State: [1] Stable
   Last error: No error

Writer name: 'SqlServerWriter'
   Writer Id: {a65faa63-5ea8-4ebc-9dbd-a0c4db26912a}
   Writer Instance Id: {4c1bdda8-b3c5-4336-9071-23568058e054}
   State: [1] Stable
   Last error: No error

Writer name: 'ASR Writer'
   Writer Id: {be000cbe-11fe-4426-9c58-531aa6355fc4}
   Writer Instance Id: {909fd0bf-f87a-4c29-8bfc-b13665c54e0b}
   State: [1] Stable
   Last error: No error

Writer name: 'IIS Config Writer'
   Writer Id: {2a40fd15-dfca-4aa8-a654-1f8c654603f6}
   Writer Instance Id: {4430004d-f3ef-45ac-be9f-abcc24acd468}
   State: [1] Stable
   Last error: No error

Writer name: 'Registry Writer'
   Writer Id: {afbab4a2-367d-4d15-a586-71dbb18f8485}
   Writer Instance Id: {d6228dae-6461-425e-8d7f-89b00e24c6ae}
   State: [1] Stable
   Last error: No error

Writer name: 'Shadow Copy Optimization Writer'
   Writer Id: {4dc3bdd4-ab48-4d07-adb0-3bee2926fd7f}
   Writer Instance Id: {993615b7-6ff2-4b00-8631-f5a13d1098f8}
   State: [1] Stable
   Last error: No error

Writer name: 'COM+ REGDB Writer'
   Writer Id: {542da469-d3e1-473c-9f4f-7847f01fc64f}
   Writer Instance Id: {41291637-333f-42d5-9092-431bf1cdf695}
   State: [1] Stable
   Last error: No error

Writer name: 'WMI Writer'
   Writer Id: {a6ad56c2-b509-4e6c-bb19-49d8f43532f0}
   Writer Instance Id: {6142ec97-1974-4440-9e7e-77c7a0fcf85c}
   State: [1] Stable
   Last error: No error

We have ESET AV installed but both the ESET and Windows firewalls are off.

I'm starting to hit a wall with this problem, everything appears to be as it should but backups are still failing.