OVM console shows black sceen

Hi,

I have:

OVS 3.4.2

OVMM 3.4.2

Problem description:

OVM Manager console view shows black screen for two Windows Domain Controller virtual machines.

1) For both affected VMs, the vnc console does show a screen shortly after VM boot (shows the Windows blues squares bitmap), but the screen disappears during boot, or turns black. The browser window indicates: "Connected (encrypted) to: vm_name." But screen stays blank.

2) For VM1, after boot, I am able to ping its vnic IP, but cannot RDP into the VM from another client.  For VM2, I cannot ping its vnic at any point, and cannot RDP into the VM from another client.