So the first results seems to be indeed pointing to a memory problem.
During one day I have left out the serial sensor. And it reported to ThingSpeak without any problems.
And the second day I have connected only the serial sensor and I logged also on Thingspeak the free Ram available in the Arduino memory. And that last logging was interesting. Over roughly one hour the free RAM in the Arduino went down from 3500 bytes to 200 bytes. And at that moment the sensor data was no longer visible on ThingSpeak (only empty fields were available in the downloaded CSV file).
So I have to check the library of the serial sensor to see what happens there.
Thank you very much for your support.