Odinen said:
I tried those things aswell, and it didn't work. So i exchanged the HomePlugs and it worked fine. Something that might work for future people with this problem is reseting the router.

Thank you very much for the help though.
Most likely one of the homeplugs was faulty.

Why use homeplugs ... they are expensive?! Better buy a wireless adapter (sort of usb stick) for about $10,- and work wirelessly.