because it happened with me once, maybeI can help.
since you have slave AFE connected to the master AFE, then you should look into the subcode of the fault in the drive monitor where if it was 1 that means fault coming from the master drive, 2 from the slave 1, 3 from the slave 2. therefore, you will focus on one AFE unit

Ithink your problem will be in the fibre optic connection on the top of the IVI board (CUSA isplugged to the IVI board). loose connection will do such problem. It happened with me whereI had broken fibre optic connection plug.

if all connections are good, your problem might be in the IVI board itself or in the IGD board (IGD is the firing gate board for the IGBT)

Regards,