It looks like the cable supplying the voltage to the SEM may be too thin resulting in a voltage drop. This could be because the SEM is supplying a lot of current
Increasing the cable thickness between Comfort and Slave would reduce the voltage drop. Also consider if you can supply the external devices from Comfort instead of the SEM so the current to the SEM can be reduced thus reducing the voltage drop
Doubling up the cable between Comfort and Slave would probably do the job
