

Are you able to check the logs at all?
This may be totally off base but: I recently had some issues with the intel networking adapter on one of my Dell Optiplexes. Still not sure specifically why, but when it was at high load (including during one backup), it was crashing. I found Intel e1000 errors on the host and was able to follow these instructions which seemed to resolve the host become unavailable:
https://forum.proxmox.com/threads/e1000-driver-hang.58284/page-4#post-302307
Perhaps a triumvirate of vets is after him.