Allright, i found the error. Our electronics guy fucked up and did pretty bad soldering at some pins. For everyone interested, the code works so far and the correct adresses for the first 3 RDAC channels are:
Wire.write(0) //channel 1
Wire.write(32) //channel 2
Wire.write(64) //channel 3
The fourth one can be found out if one looks at the bit composition of the instruction byte in the data sheet. The instruction byte is effectively the byte that is send to choose the internal channel.
I can only tell you that the number is in the range 0x2C - 0x2F.
is unfortunately wrong
Can be closed. Thx