Sorry for the very late follow up. Health problems and time shortage.
What I see is that this happens sporadically and (so far) never twice in the same movie. Also, when I "rewind" the movie to a spot before where the blackout happened and restart playback, it does not happen again at that timestamp.
I recently noticed the blackouts also happen when walking through or looking at the interface (like looking through the listed movies).
As for your questions:
Does it only happen once for that particular video or multiple times during playback ?
So far, only once.
If only once does it happen predictably just a few seconds after starting playback, or is it in a random location ?
It looks random, I've never seen it happen within a few after starting playback though.
Do you have "adjust display refresh rate" in Settings->Video->Player set to always or on stop/start ? If so does it still happen if you set that to Off ?
I have not changed that setting from the default. I'll look into at tonight.
A couple of osmc updates have been (automatically) installed since my first post. Nothing changed with regards to the blackouts. I did notice that during the first 2 updates the TV showed "Unsupported resolution" (or something like that). The updates seemed to have continued anyway since after a few minutes OSMC rebooted like normal (without my interaction). The updates after that all showed the normal blue ascii like screen.