I have this exact problem as well, just updated from 6.8.3 to 6.10.3 and now my Plex VM with a GTX 960 pasthrough won't boot.
When set to VNC + GTX 960 it sits on a black screen and Windows doesn't begin to boot.
When set to VNC only Windows boots and acts normally, Event viewer shows nothing logged during the time when it was 'started' with the 960 or 960 + VNC, safe to assume that Windows doesn't complete boot but also don't Blue Screen.
SysConfig:
Intel i7 6900K no OC
64GB RAM
Unraid PRO
VM: Win10H, 24GB RAM, GTX 960, 14 cores, SSD Image location