Just now I think I found the culprit: OpenTrack/AITrack...
Although previously I wasn't running them, I did have them open (I thin in case the battery of TrackIR ran our).
Tonight again it didn't work, but then I checked if maybe there was a conflict with Opentrack/AITrack, so I switche dthem off. Then switched off TracKIR, switched it on, and started DCS. And voila, the second green light cam eon!
So lesson learned: Charge your batteries, and don't run other trackers!
BaronVonVaderham's post in TrackIR problems was marked as the answer