It is me again.
I just couldn't stop the investigations and now noticed something!
Actually it seems the navi buttons HAVE something to do with the problem, but in another way as I thought of:
Because what just happened was: By "reconstructing" my scene I made a mistake and assigned the button textures from the wrong atlas to the navi buttons, means they now didn't use the same material as the other sprites of the scene any more. Now after the video finished or was interrupted the sprites all looked correct. The same is the case, when I don't have navi buttons at all. (The two navi buttons sit in front of everything all the time: during the video is playing and afterwards in front of the other sprites as well.)
Is it possible, that in the moment, the video is done or interrupted = texture killed, that the system sweeps all the stuff from memory, that is just visible in front of the video plane, and thus destroys the material of the navi buttons? Well, this would probably be a question for prime[31].
I still wonder, why this should be the case. Also why this is not happening, when there is a unity sprite with the relevant texture hanging around somewhere "behind" the scene.
Another thing I observed: The materials get indeed not messed up that often, when I let the video finish itself instead of interrupting it. I should address this to prime as well.
Still everything quite obscure to me.