So I think I am having a similar but not exactly the same issue as the one about blank/black screen on start:
Theater loads and connects fine, but if I let the Emby screensaver start (either the logo or backdrop) I have to use the mouse to "wake it" before I play a movie or else it just plays UNDER the screensaver - and since it is a dedicated Media Center computer that means VNC - where it didn't do this before (about the same time that thread was started, somewhere in August). Also at the end of playing a file it looks like Theater doesn't stop/close the file or player. It sits at a black/blank screen instead of going back to the front page of Theater. If I play another file while on that post-file black screen there is no problem, it just switches the player right to it and goes.
This behavior is NOT seen when using the browser, only Theater.
I tried the solution in the other thread anyways and it didn't work. It is almost like Theater isn't sending the proper end signals (screensaver and file/movie)...?