@mfalkvidd Thanks, that solution worked and sorry I should have found that posting myself of course.

FYI, for loading the ESP8266 Board definitions the usual URL https://arduino.esp8266.com/stable/package_esp8266com_index.json did not work for me anymore. Instead I had to use https://github.com/esp8266/Arduino/releases/download/2.4.2/package_esp8266com_index.json

@mfalkvidd In my gateway hardware setup nothing is connected to GPIO pins apart from those used for radio connection as described in https://www.mysensors.org/build/connect_radio .
Next to that, I have uploaded the sketch to 2 different NodeMCU-boards with absolutely nothing connected.

I have also successfully uploaded the gateway sketch from two different Linux computers with same negative result. Only the serial monitor does not work in a Linux-ESP8266 connection as Linux apparently does not support the rather uncommon 74880 Baud rate which is forced by ESP8266.

The funny thing is that about two years ago, using exactly the same hardware, I managed to set up a fully functional gateway with a remote Arduino-Nano temperature node and a visible, static-adressed LAN-connection. As it would not connect to my Domoticz-system at the time, I left it for a while until I tried to revive it recently.

Blush, blush. Thanks.