Yay… problem resolved…
I found, I needed pull-up resistors on the SCA/SCL lines… now its working 100% 
edit:
quick note, for others that might need this, you can indeed set SCL/SDA for a different i2c bus,
e.g. to use i2c4, just add this before Wire.begin()
Wire.setSCL(D13);
Wire.setSDA(D14);