Great, yes indeed, that did it.
And now for the next problem: the Sony TV can play back various (transcoded) movies, but has no sound. The videos definitely have sound. In fact, putting one of the examples on a USB stick and plugging it into the TV directly, it plays it back just fine, even in .mp4 format with h264 video and aac audio codec inside (which always makes me wonder why transcoding is needed in the first place but I understand that's to do with DLNA encoding rules rather than hardware codec capabilities). I guess you would need more information to diagnose this. The selected device player is 'Sony TV (DLNA, 16:9, 1920x1080)'. The TV is KDL55NX813. I have tried switching the transcoder codec from AC3 to AAC but that made no difference. Strangely, with a completely different TV, the transcoding plays back with sound on the TV.
Any hints? I appreciate that this is tedious, so worry not if you have more urgent things to do.
Seasonal greetings.