Sorry, that I have to answer myself. Cannot edit the post above.
After some google, I found out the i2c behaviour seem to have any issue with a pulled down SDA signal.
Or a flipped SDA / SCLK. In case I flip, all adresses are zero. So this was not the rootcause
Unfortunately, I am now at my parents house and cannot access a scope to see the signal on Pin3.
In worst case I just got delivered a faulty OLED display by amaz*n.
In the past, I had connected a normal LCD, where at least the i2c detect worked well and served a single adress.
So I would assume, the RPI board is not the rootcause.
Maybe I give it a try with a SPI connection.
However, it doesn't explain, the issue when I try to install the OLED addon script resulting an error.