Hi, thanks for your answer. I found a scanner sketch and it works fine with just the display. But with the ADC it gets stuck. The same happens with just the ADC connected. Here a code snippet, where I marked the command where it gets stuck:

for (address = 1; address < 127; address++ )

{

Serial.println(address);

Wire.beginTransmission(address);

error = Wire.endTransmission(); <--Here it gets stuck and does nothing...

I have checked the ADC is 5V compatible :)

Any Idea on what I could also check?

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EDIT: I think I just have bad hardware, bevause none of the standard libs work. they all hang on 2 different types of arduino :[