I flashed the QMK firmware and something went wrong - no idea what. This is described in this thread that was already linked but somehow the Forum software removed the Link.
Anyhow, after unbricking it by using a USB-UART Adapter and a flat ribbon cable + breakout board I flashed again and everything worked. So just bad luck on the first try I guess.
As long as you don’t want to flash alternative keyboard firmwares, you should be fine. And in any case - it is possible to recover with some (cheap) additional hardware.