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.