Don’t know exactly how proxmox handles it, but there may be an interface for bridging from physical host to virtual machines. In other vm environments I know better, this interface could make problems.
So Nethserver is just not available anymore but proxmox still reachable and internet without Nethserver too? Looks like network config or/in virtualization problem…
Thanks, I think they may help a lot.
And please don’t forget: 
And the network configuration of the proxmox interfaces would also be interesting…