For documentation purposes, let me make quick summary about the private emails I have exchanged with Mikael.
First of all, this post is related to the "Antenna Swich AS 1289" not the the Webswitch. Mikael explained that the control board is the same. However, of course, the firmware is different.
I checked with him that I have corretly connected the sensor to Bus B. The AS1289 comes without connector for this purpose. I have soldered a connector to make the sensor connection easier.
Mikael tested the sensor in-house and he confirmed it should work. However it did not work for me. So the conclusion is that I am using a counterfeit DS18B20, which seems not to be 100% adehent to the specifications. Per Mikael explanation, a real Dallas device must be used.
I checked on the Internet that the majority of devices offered on eBay, Alliexpress and similar are chinese counterfeit DS18B20.
Well, at this point I am in a quest to find a real DS18B20, which seems only to be available in the distrubutors like Digikey & Mouser, far from my reach.
John (AD8J) thanks for your input. Mikael, I really appreciated your attention to this matter. Fred PY2XB