Thanks for the reply! I do have a pullup resistor between SDA and VCC and SDL and VCC. I use a 4.7k ohm resistor for both (as can be seen in the wiring diagram). I'll try another library, and try the code you sent! It will probably be tommorow before I can test it, so I'll post the results of the test tomorrow.
The test:
Use this library and the code from Koepel.