i finally got it working!!
The magic is that the AD5293 uses SPI Mode 1 (CPOL=0, CPHA=1).
So i simply had to add
SPI.setDataMode(SPI_MODE1);
Finally the working code Looks as followed:
const byte AD_CS_PIN = 20;
//Start SPI Stuff
pinMode(AD_CS_PIN,OUTPUT); // SPI - CS
SPI.begin()...