I reinstalled the original U-Boot firmware to see if the error was related to the custom sd-card capable firmware. Furthermore I created a Debian usb-stick. Still no luck.
With an usb stick inserted, u-boot gives me this messages.
USB: scanning bus for devices... 1 USB Device(s) found.
scanning bus for storage devices... [b]0 Storage Device(s) found.[/b]
Please, does anyone have a clue?