Update... Solved!
Again I got the entry:
2017-09-14 08:40:22.230 MySensors: Gateway Version: 2.1.1
2017-09-14 08:40:24.215 MySensors: Gateway Ready...
in the log.
I found that I had a broken sensor. When checking the sketch for this sensor
i noticed that I had enabled repeater function. After removing this faulty sensor, there are no more
spamming in the Domoticz log file.