MK3 does not start printing if nozzle temperature is set to 240°C

I use a MK3 an printed PLA (210°) in the past. Now I like use PETG @ 240°C. When starting printing printer heats up and moves x to the right, than immediately stops (seems to get a reset) The temperature drops…
I tested several times, always the same. What can I do to get it work on high temperature ?