It seems this issue is related to the CPU mode with Win2016 and Win10 on KVM. (Synology uses KVM underneath).
You can check the current CPU config when you login e.g. via SSH, then identify a (running) VM with virsh list and virsh dumpxml <ID>.
My vCPU were on "passthrough". I temporarily solved this issue by joining the VMM Pro Beta Program (under License) and then activating the CPU Compatibility Mode (VMM Pro only) for this VM - this sets the CPU Fallback to core2duo, which works fine for me (Win2016 and Win10).
KR,
lousek