Update: I’ve been able to isolate this issue to my multi-monitor setup. With both Cubase 9 and 10, the video window works fine as long as it is on the primary display. When I drag it to my secondary display (which has a different native resolution than the primary) it works fine until I close the video window and attempt to reopen. Then Cubase hangs. If I try to load a project that’s been saved with the video window open in the secondary display, the project immediately hangs.
The short term fix for this is to physically disconnect the signal cable for the secondary display and then load the project. Cubase will drop the video window back in the primary display, and everything is fine. You can then reconnect the secondary display - just don’t drag the video window over there and forget about it. I hope this helps some of y’all.
I have a ticket on this with Steinberg support, and they have been very responsive so far. I am hopeful that a long term fix is on the way.