You can try to clear internal memory of Your Cybook the way I did. Just downgrade Your device to 796fmt version. This should delete everything from Your Cybook. After that, instal again firmware that You like. If this will work, try to check if everything is ok before You put SD card in the reader.
796fmt firmware is here:
http://bookeen.free.fr/bin/CybookGen...nelb796fmt.zip