rico72 said
I'm trying now to remember what I have changed (it worked great for almost a year)... the one thing that pops up is that I have added a serial-sensor as well. Directly connected to one of the serial inputs of the Mega. Is the ThingSpeak Arduino library using serial bus somewhere?
Can you try reverting to your old Arduino sketch (without changing any hardware connections) and seeing if the problem still occurs. That will help isolate if it is a software issue. Next step would be then to go to a setup without the new sensor and seeing if that configuration of old hardware + old software gets you the correct behavior. The only reason I suggest this is to rule out hardware interrupts from the newly added sensor messing with the data transmission to ThingSpeak.