So randomly, just now, almost as if they knew we were talking about it, the “track is not playable” SoundCloud tracks began working again. This is both good because it’s nice when things work, and bad because it happened in the middle of me just getting in to diagnosing the issue, which at least for now is no longer reproducible.
I know it wasn’t something unique to this particular Pi or network; we saw SoundCloud playback begin failing at the same time in the past week and then just now resume on ~10 devices in 5 different locations, and also it looks like somebody else here presumably ran into the same issue (I can’t get mopidy-soundcloud to play anything). Also it didn’t seem to be related to the SoundCloud users’ track settings; these tracks were from multiple users, including myself, and I know at least that I did not mess with anything on my SoundCloud account while this was going on.
Very weird. It almost feels like some dev at SoundCloud silently broke something by accident then fixed it.