Old topic, still unresolved: Tradfri motion sensor with Hue bridge does not work

Hi there. I was able to connect the Tradfri motion sensor to my Hue bridge (no IKEA bridge) following some of the many Youtube instructions on the web. Seemed to work fine, LED flashed as expected. Then connected the motion sensor to the light bulb (also Tradfri) - flashed 10 times, looked good. But: Now the motion sensor just does not work. I can jump in front of it, in the dark or light, nothing happens. Tried with two motion sensors in two different rooms, with two different light bulbs. Same problem.

I had this already working about a year ago with those very motion sensors and light bulbs, but since the batteries were low and I had to replace them it will not work anymore. Reconnected the whole stuff like 20 times now, still nothing works.

Are both motion sensors just broken? I'm considering throwing them in the trash, rage-quitting, tried this reconfiguration for weeks now. What is the problem here? Out of ideas what to try.

Can anyone give a hint?