we have a BE server which backups 4 MS Hyper-V servers and a lot of Windows and Linux VM's on these servers.
Normally this works just fine, but every few weeks the backup of some of the Hyper-V hosts just get stuck at the backup system state.
There is no longer any progress in the backed up files/sizes and the througput of the back just drop and drops.
Attached an example of today, where the backup just stuck at 7GB in the system state, the job is already running for 9:33 hours and nothing happens any longer.
And the same for the second Hyper-V server, already running for 9:03 hours, 2.75GB backed up and also stuck
Also a stop of the backup does not work.
We use BE 21.0 1200.2536 and the latest windows 2019 updates.
We have seen this many times over the last 2-3 years.
I have seen that some of the vss writers on the source server are in status 5 waiting for completion.
Do you see them too?
vssadmin list writers
They should usually all be in state  stable