Currently I am consolidating my environments and one of things I am doing is moving VM’s to new or different hosts.
I moved my Pastel server, started up the machine and no network connectivity, however a few minutes ago I just had.
Upon investigation the Microsoft Hyper-V Network Adapter had a yellow exclamation on it with error code 31 that windows could not load the driver for this card.
I then shutdown the machine and added a legacy NIC with the same result. I then ran SFC /Scannow and windows reported no errors.
The only real fix to this is adding a VHD (Virtual Hard drive) with updated NIC drivers and then updating it or re-installing the server. Quite painful as you are pretty stuck without a connection and especially that you cannot add USB devices to Virtual Machines without making them pass-through disks but not all of them can do that.
Another thing is that Windows Server 2012 has the Integration Components built in so you cannot remove this and re-install it.
Hope it helps