Hi there
thank you eselarm for your valuable input, eventually I managed to login using a custom configuration with Armbian Image. Maybe during the previous flashing there was another config file with a different root password, which I did not notice.
However, according to https://docs.armbian.com/User-Guide_Getting-Started/, an initial login for root should be possible with password 1234?
Anyway at least its working now, thanks again and thank you Guation for the custom image!
Domillo