Redundant Controller Failure

Hi guys,
This problem relates to Yokogawa Centum VP DCS.
My Redundant (secondary) controller fails after few days. Controller model is CP451-50 S2. It gives message of "0303" FCS Fail. (Selfstop code= 5800 20F8 0001 2700). I have to reset the controller module manually every time, after that it comes to standby state.
What could be the reason of failure???
Yokogawa reference guide says it is a communication failure.

Some anomalies in the DCS that I want to mention are: 1 Node of the DCS is missing the redundant SB401-50 S1 card. Also the redundant ESB bus coupler modules are of different suffix. One is EC401-10 S2, while the other is EC401-10 S3.
What should be done to keep the secondary controller stable??

Helpful replies are welcome.