@mfalkvidd @electrik Yes! This did the trick for me. Thanks for your guidance.
Probably the instruction to use a version not higher than 2.6.2 has been given somewhere/sometime and I have overlooked it.
The serial monitor remained blank during upload, but when I select 9600 baud and press the reset-button it gives what I think is a healthy response:
⸮⸮5R⸮⸮64 MCO:BGN:INIT GW,CP=RNNGE---,FQ=80,REL=255,VER=2.3.2
174 TSM:INIT:TSP OK
state: 0 -> 2 (b0)
state: 2 -> 3 (0)
state: 3 -> 5 (10)
connected with XXX, channel 6
dhcp client start...
403 TSM:INIT:GW MODE
458 MCO:REG:NOT NEEDED
3629 GWT:TIN:IP: 192.168.178.4
3680 MCO:BGN:INIT OK,TSP=1
3708 TSM:READY:NWD REQ
pm open,type:2 0
And sure enough, the LAN has detected an ESP device at 192.168.178.4 now.
Secretly, I was hoping that connection to Domoticz would now also work but unfortunately I keep on getting the persistent message:
2022-11-25 11:54:58.598 Error: MySensorsGateway: Can not connect to: 192.168.178.4:5003
However, that probably needs to be adressed in a different topic.