It looks like the RTL8195 has 3.3V IO. The potentiostat shield powers the MCP4261 with 5V. So you are connecting a 3.3V device to a 5V device - I think. I'm not sure if the IO pins on the RTL8195 are 5V tolerant, but you might want to check. Could be related to you issue.

Note, the MCP4261's could be powered off of 3.3V. They have a pretty broad allowed input range 2.7 - 5.5V. This would require cutting one trace - the one going to the 5.5V on the shield - and rerouting this trace to to the 3.3V pin.

The spec sheet for the MCP4261 can be found here. http://ww1.microchip.com/downloads/en/DeviceDoc/22059b.pdf