Unfortunately I can't even reproduce such problems. I can toggle the screensaver all I want, but Theater View always comes back fully functional.
Interesting. It's 100% reproducible on my living room system (which had the screensaver set to engage after 1 minute of inactivity), which is running Windows 10 with an NVidia 960 GTX running the latest stable drivers. I have a few other NVidia systems in the house, so I'll try some testing and see if I can figure out any differential.
What's more interesting to me is that I didn't get full locks until more recently. Prior to that I would still get the error on resume, but it would flash and vanish, but lately it's been "sticking." I think it followed the same trajectory described by Ashfall above:
Beginning with version 21.0.76, when returning from screensaver the entire screen goes black with a white text error message in the center "Error drawing using Direct3D (device lost)". There is no exit from this screen without a keyboard plugged in, so I have to reboot.
Versions 21.0.54 through 21.0.72, when returning from screensaver the same exact black screen with white error message only flashes very quickly and then the normal theater view screen is displayed.
Is there anything that changed between 72 and 76 (or thereabouts) that might've made the error "stick"? I didn't see anything likely in the release notes, but I'll attempt a bisect when I get home. I can live with a flash and gone, it's the hang up that's tough.
Meanwhile, I've got a hunch about some things that might be related; I'll see if I can track down some kind of difference.