Same problem here, after PBS Update to 4.1.0. MTU 9000 - 10 G Network.
root@pbs:~# uname -a
Linux pbs 6.17.2-2-pve #1 SMP PREEMPT_DYNAMIC PMX 6.17.2-2 (2025-11-26T12:33Z) x86_64 GNU/Linux
vzdump 103 101 --notification-mode...
I am having the same problem after upgrading PVE to 9.1.2. It seems to only be happening to Windows VMs. Backups are causing the VM to halt after which I have manually qm unlock, stop, then start, to get them responsive again. My PBS is on 4.1.0...