VSS - The semaphore timeout period has expired

On a Server 2016 box Windows Backup fails with

The backup operation that started at '‎2016‎-‎12‎-‎23T02:00:13.001000000Z' has     failed because the Volume Shadow Copy Service operation to create a shadow copy of the volumes being backed up failed with following error code '0x8078006B'. Please review the event details for a solution, and then rerun the backup operation once the issue is resolved.

...because VSS failed with

Volume Shadow Copy Service error: Unexpected error DeviceIoControl(\\?\Volume{aa36bda0-318c-4fea-a23c-69570ca79c35} - 000000000000021C,0x0053c008,0000024028BF30A0,0,0000024028BF40B0,4096,[0]).  hr = 0x80070079, The semaphore timeout period has expired.

This has been running normally until recently, backing up to a NAS.

Other Server 2016 boxes are still successfully backing up to the same NAS

There is well over 3TB of free space on the local drive and on the network NAS

Have banged my head against this one for a couple of days, so any pointers would be very much appreciated.

Thanks

============================= UPDATE ========================== I have noticed that this server has 64 previous versions (Local Drive > Properties > Previous Versions) where all other servers show 'There are no previous versions available') and appear to be twice a day from 29 September to 29 October.

Tried to delete some (vssadmin delete shadows /for=c: /oldest) but that has no effect at all. Must have set-up something with this server that I didn't with the others but no idea how I did it :-(

In the end, as there was little activity (Christmas eve), I restarted the server. After it booted up I was unable to get back into it with mstsc or even a local monitor/keyboard/mouse so just switched it off and back on again (crossed fingers) - then I could access with mstsc and backups started working again - there were a few red boxes with Server Manager disliking the off button being pushed but it would appear no major issues

I hope this is not an omen of things to come with Server 2016 :-(

Like any other business we rely upon these things being up and running, and backing up, other than scheduled maintenance after patch Tuesday!