@György-Farkas Also it seems that the atmega is just connected to the 3,3V and not the 5V, and maybe it therefore was protected since it never was in direct contact with 12V.
I hope at least

Thank you for your helpful answer, I will check for short circuit.
Do you have a suggestion for any other
protector IC at the USB host port. It is impossible to see its typing.
/ Lars

I managed to fry my Arduino dock using a 12V supply even if I had read before that it just takes 5V.

My Onion2+ and the OLED expansion is not broken.

However. Which component get damaged. Any suggestions?
The level-shifter, the regulator or what else?

I would like to repair the unit if possible.

Regards Lars