Eval-ADUCM355QRSZ problem with cyclic voltammetry

We have the evalutationsboard ADUCM355QRSZ and succesfully tested the first example programm. 


While testing the cyclic voltammetry with the recommended star resistor network, we encountered some problems. Vzero is influence by Vbias. While measuring Vzero at SE/WE electrode we discovered  that Vzero didn't behave as expected. Instead of two constant voltage values Vzero is changing proportional to Vbas. 

Image of voltage on the SE/WE pin to ground. We didn't change the programm . We only change the cycles. How can we solve this problem.