There were no USB devices connected but it was in fact a power issue. I had my raspi connected to a simple USB charger next to my desk which probably did not supply much more than 500mA . As soon as I connected a real power supply with 1000mA things turned for the better (sadly, that one burned out shortly thereafter - not my lucky day). Still funny that the device came up at all without enough power, that took me a few hours of toying around, downloading all the other images and retrying over and over.