After just a few flights, the FrSKY telemetry of my IRIS suddenly stopped working. The Taranis was not updating the display or it was showing nonsense values, like altitude -65535 or similar. Time to do some diagnosing.
After the suggestion of a friend, I checked the data stream with a logic analyser on the Pixhawk and on the D4R-II and found that at the D4R-II there were lots of framing errors and also the levels didn't seem right. Luckily, I had another level shifter flying around and made a new cable, with which the telemetry worked perfectly again. Seems, the level shifter had burned out for some reason.
All in all, diagnosis and fix were alltogether less than half an hour.