So this is indeed some underlying issue, since the exact same LED (well, a different LED in the same spot) blew its dataline again. I’ve asked what this could be on /r/askelectronics. Most responses were not very helpful and there was some annoyed patronizing, but I think the most promising suggestions were that it may:
Meanwhile I also designed and ordered a drop-in replacement for the level converter based on a WS2812B, which arrived last week:
After assembly I installed it in the controller and it seems to work just fine:
So, next steps: check that cable for a short. If that is not the issue, maybe I’ll replace the LED that keeps failing with a series-resistor to reduce the ringing in the cable.
To be continued. Again.