Eval-ADUCM355QRSZ problem with cyclic voltammetry

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

https://www.analog.com/media/en/technical-documentation/user-guides/EVAL-ADuCM355QSPZ-UG-1308.pdf

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. 

Thanks,

Trung