Error 39520 on Multi Robot System

Hi Everybody.

   I am new here, and would like to listen to good feedback for the error.
   Recently, I have commissioned the multi robot system with single controller.
    ->  IRB 4600-40/2.55 Type C
    ->  IRB 1200-5/0.9 Type B

   The error 39520 has occurred from the beginning when I started commissioning from our workshop. 
   Robot has been installed at the customer site and Alarm kept occurring and ABB Service 
has replaced 
     - Ethernet cables (  cpu - router , router - Axis Computer 1, router - Axis Computer 2 )
     - Router 
     - Computer Box
     - Power Supply 

   Now, still the alarm is happening once a day and someday 3 times a day.
   This error happened randomly like no production but during the night happens and sometime while on production and unable to find the any pattern.
   Sometimes Axis computer 1, Axis Computer 2 or both of them.
  Connected Earth cable correctly ( Nothing changed )
  This is the diagnostic link.  

  Any Feedback will be welcomed.

   From Tony Kim