Today I tried the following things without success:
- Resoldered all pins
- Connected the sensor board to 5V and 3,3V of my nanowii board
- Connected the sensor board to 5V and 3,3V of my raspberry pi
- Enabled / Disabled internal pullups of the nanowii board
- Measured the voltages on the sensor board (3,3V connection => 3,31V VCC, 3V on SCL and SDA)
- Measured the capacitors on the sensor board => 3,06uF and 0,20uF (Datasheet says 4,7uF and 0,22uF could this be a problem?)
If no one has another idea what I'm missing, I think I will have to buy a new one...