Guess what? It must have been a wrong cable in a USB 3.0 port. Changed the USB-cable and put it in a USB 2.0 port (on the same computer) and it flashed flawlessly. Thank you guys for your advise on this problem. So new flashers beware: use a proper USB cable and a USB 2.0 port!If nothing works, I would try to flash an official rom version via Miflash without relocking the BL and from there try to flash xiaomi.eu again the way you did.