Hurm strange things are happening here ...
short timeline:
did some googeling
most reasonable result: sketch is too big and conflicts with bootloader
tried to reburn bootloader via usbtinyisp and arduino software
can't upload anything to the arduino:
avrdude: stk500_recv(): programmer is not responding
Oh NO, Not again!
got certainty, that this error results from "corrupt" bootloader (I already "lost" an uno with this problem)
tried to burn the bootloader via avrdude:
I used the config file from th arduino software AND the .hex file from the arduino software
result: can upload again AND the mqtt sketch works too
I don't have to understand that, have I ?