@Nca78 Yes, that should have been clear to me by now.
I just lost some time trying to burn the boot loader as always I got this error:
avrdude: stk500_getparm(): (a) protocol error
Checked, tested and even redid the soldering to exclude any wiring problems. Turns out adding a cap (0.1 µF in my case) between the programmers GND and RST did the trick.
Thank you all!