I have a similar problem. At the start of a day some times, project will load fine. But Program monitor is white. Audio will play but the video does not.
Win 10, drivers up to date. 2 monitors running off a USB-C port on an Alienware R5. Laptop closed. So just the 2 Dell monitors. No connected audio input, headphones connected to headphone jack on laptop.
Only way to solve is repeated reboots. took 3 this morning. Then fine the rest of the day.
Using custom editing workspace with program monitor on second monitor. But have same experience with factory workspaces.
Once this occurs, and I quit the app, it will not restart without a logout or reboot.