OMG. I finally found out what the problem is!
The device has manual dials to set motion sensor time and light sensitivity. If they are on the minimal setting (“T”), the device is put into “Walk Test” mode, in which the light always go on for 5 seconds, irrespective of any other setting. That’s why the Z-wave settings would not work. After moving the dials away from the “T” setting, everything worked as expected!
The manual should really make this clearer. I did find one table there that mentioned something about the different modes… I should have had a closer look before. I just couldn’t imagine it. I had somehow expected that the Z-wave capabilities of this device would be able to override the manual settings, and that’s why I didn’t even touch the dials. This took me forever to get sorted out!
I guess the conclusion is RTFM!!!