@mbj No problem! Actually, I've encountered another issue. But I found that that may depend on the bootloader. The first time I "solved" the problem was another board with a generic 328p breadboard bootloader (I apparently didn't pay attention when burning the BL for the various boards) Last week I had a problem with a board with the Gert Sanders 328p bootloader when I couldn't enable the MY_DEBUG. When I changed the board type it compiled, but wouldn't upload because of the differences in baud rate. I will probably have to reflash al boards withe the same bootloader but that means de-solder all radios ... When I have time again I will investigate further.

@gohan I live in Switzerland, but perhaps my place has more humidity than normal. It's not silly problems we're working on, we're learning all the time!

Lovely work. Are you proposing to sell the boards? SMD looks too hard.

I took a drastic measure, uninstalled the whole lot, deleted all libraries, rebooted the PC and re-installed Arduino IDE, 1.6.12 now, installed MySensor-Master.zip, added the SI7021, ash204 and RunningAverage libs, from https://github.com/mysensors/MySensorsArduinoExamples/tree/master/libraries and guess what, still all the familiair error messages. When I took all the SI7021, ash204 and Flash items out it compiled nicely... My next challenge will be to add DS18B20 temp sensors, add the furnace controller logic (which works nicely on an Arduino Uno on the moment) and make it all visable in Domoticz.