looking for Schematic of REK RK8511 Electronic Load (Manyuo M9711, BK8511)

[Xoneoo]

Hello

I search for a Schematic of the Electronic Load REK RK 8511 it's similar to Maynuo M9711 , or BK 8511.

My Problem with this Unit is, the shown voltage on the Display and on the PC-Software too is ~2V below the real Voltage without any load.
But when I use the Rear Terminal the Voltage is only ~1V below and when I connect the Frontpanel und Packpanel Ground ( and switch to backpanel) the Voltage is right!

Perhaps anyone can help me.

I have checked the PCB and the Relay but I cant find any issue, I think a Schematic can help me very much.
Thank You!