ABB PM861 with redundant configuration

Hello to all

Our factory uses PM861 with additional configuration and bc810. When I tried to download the edited program, both CPUs went into error mode (flashing light error). While both bc810s worked normally, but all LED faults related to communication module were blinking and communication disconnected. Therefore, after resetting both cpu, the error was corrected. But the program was removed from the cpu and control builder prompt that need to cold download.
Does anyone know the cause of this problem and what solution should be considered so that the problem does not occured.
Best regards.
Is it the first time you are downloading to the controller ? Can you check the termination of CI bus is proper. check the RCU link cable between both controllers are tight. Also check the RCU link cable between both BC810's are tight.