7i96s with custom board connection error

Hello people, I am currently using a custom board which is connected to my 7i96s via the serial interface.  When I switch on the drives, I often get errors like the one in the picture below: the connection to the additional card has been lost. I don't know what to do next. I use the latest version of LinuxCNC 2.9.3 as the operating system

Sounds like a EMI / grounding issue.

(the first error means that the card did not respond to a request)

Is the serial connection differential?

If so is there enough noise to exceed the common mode range of the serial interface?
(7I96S RS-422/485 port common mode range is -7 to +12V)

You might try  a common mode choke (large ferrite bead) on the serial cable