Hi tomi,
try changing your software sources download location and updating your system again, if you are unsure how to do so, read the update guide for more info:
These are my own setting preferences, use at your own risk!.
AN IMPORTANT NOTE FOR LINUX BEGINNERS. WHEN YOU ARE IN RECOVERY MODE, THE KEYBOARD DEFAULTS TO US ENGLISH, IF YOU HAVE ANY OTHER LANGUAGE KEYBOARD OTHER THAN US ENGLISH, WHEN YOU TYPE "Y" (FOR "YES"), IT MIGHT SHOW UP AS A DIFFERENT LETTER, ON MY KEYBOARD I MUST PRESS THE "Z" KEY WHICH EQUATES TO "Y" AS I HAVE A GERMAN KEYBOARD, JUST PRESS "BACKSPACE" AND HIT A DIFFERENT KEY UNTIL YOU SEE THE "Y" SHOW UP!.