Time Capsule in the past has had many problems with failing hard drives. This is likely what has happened, and I would recommend that you call Apple to confirm that this is the issue, and so they can send you a replacement Time Capsule. Even if Time Capsule is out of warranty, explain that it is unacceptable for a product like this to fail after such a short time, and that they should replace the product for you. Their phone number is 1-800-APL-CARE.
So I tried some things and found out:
I can read all the files on the TC: music, movies etc. everything can be played.
I can also copy duplicate files on the TC.
Transferring files from the TC to my Mac also works (1 GB took 7 minutes)
but from my mac or another hard drive to the TC just doesn't work -.-