mit folgender Anleitung konnte ich das ganze Gerät erfolgreich reseten:
1.Press power button and volume+ button till you see Phone and USB cable picture.
2.Install lastest version of Chinese Mi Pcsuite (Don't use English version as it has some issues detecting your phone) from http://zhushou.xiaomi.com/.
3. Now connect your phone to pc via usb cable
4. A popup menu will appear in Chinese. Select the second option which is factory reset.
5. After that the phone reboots and everything will work fine. However you will lose user data.