OK, here's current state of play!
When I select a video in the SLR app to stream, it gives me a few seconds of black screen, followed by the orange popup message "Error P)laying Video: Unsupported Format."
This happens on every gay/shemale video, and occasionally on straight. Occasionally, when playing a video that does actually work, the video is heavily pixellated, and usually crashes after a moment, giving the same error message.
Swicthing to DirectShow has not made any difference.
The same videos will stream on the website without an issue. Using "Watch in SLR" however will give the same errors a running the video inside the SLR App on Oculus.
I have downloaded videos that would not stream. As downloaded videos, they work perfectly.
I have installed all the codecs suggested - no change.
What is especially frustrating is that this is happening on videos that I have watched streaming through the SLR app in the past, and they worked without issues then. Something has changed somewhere, and I cannot imagine what it might be.
I will send the output log as requested!