[popcornmix]

Running that command gives me just

PCM supported: Max channels: 2, Max samplerate: 48kHz, Max samplesize 16 bits.

, on OE 3.2 as well as OE 4. Which would imply that it never worked in the first place... Which is strange, because when I started enabling that in the past, tests I performed back then where giving me a positive outcome that passthrough was working.

Yes, no DTS or AC3 passthrough is reported as supported. Frodo builds would have just behaved as if passthrough was disabled (downmixing to stereo).
Disable passthrough for the same behaviour as Frodo.

Here's a log where I try to use "Play using..." to start playing the movie on my TV through UPnP: http://xbmclogs.com/show.php?id=150403

I'm trying it with 2 different movies. Both end up with nothing. My TV, however, can play the same movies just fine when starting them either directly on my TV as well as through "AV Control Point" on my desktop machine. The latter functions as a UPnP control point as well.

Unlikely to be a Pi specific issue. Can you try the same with xbmc installed on another device (e.g. window PC).
If that fails, you are better off reporting this is a general xbmc thread (I know nothing about UPnP).