Just found the problem.

In the devices tab there are two colums: IDX (column 2) and ID (Colum 4). Guess what, I configured the value for ID in the ESP instead of that of IDX ...

It is working now!

I have a DS18B20 connected to an ESP8266 (ESP12F; running the ESP Easy firmware). The ESP is spitting out temperature readings from the DS18B20, but they are not appearing in Domoticz (Version 2.3530; running on a raspberry Pi). From a tcpdump I can see what command the ESP is sending to Domoticz ...