Alex - did the emulator resolve this for you?
I am seeing the exact same issue. All was working normally, then I left for a few weeks of vacation, return home to a lot of Win10 updates on both PCs, and now, my secondary machine MUST have a monitor plugged in, or the primary gets a black screen.
The connection stays active, but the screen is black. Key strokes still transmit to the secondary from the primary. There is just not video appearing to the primary - black screen.
If I connect with the primary to the secondary while the secondary has a monitor plugged in (HDMI), KVM works normally. If I unplug the monitor from the secondary, it stops updating video in the primary's KVM window. If I plug the monitor back into the secondary, all resumes normal functionality.
Is there no setting anywhere to adjust this / correct this? From what I can see, this breaks Multiplicity entirely for me, and I suspect we aren't the only ones.