I tested another i2c device (EEPROM) and got the device address detected. So i guess it has to be a hardware problem.
As far as i could find out:
- RPi has a 1.8k pullup resistor on SDA and SLC already on board
- GY 521 also has also a pullup resistor (2.2k) on each line on board
(The wiring should be ok, there are a lot of instructions around the net and i've checked it multiple times. I use 5v on the GY521 since it has a voltage converter)
Any help appreciated!