Well... as it didn't finish to backup, I stopped it and with disk utility I tried to repair the disk. After some minutes, it said it couldn't be repaired, that I should format the drive. So I formatted my time capsule and started to back up all my Mac from the beginning. As approximately 80GB were already backed up, I got this message, saying that there was an error, that some files couldn't be backed up, to try again later or repair disk if the problem would persist.
What could be wrong? What should I do?