Ethernet Backhaul kills LAN traffic *partly*

That's exactly what I already tried as explained in my first post... 

Ok, so you guys need a picture? Ok, here you go... :D

The orange and blue LAN cables are the 2 alternative connection methods that I tried. Orange is the first method, where I simply connected the RBS40 to the existing 8-port hub in the living room. Blue is the 2nd method where I unplugged the cable coming from the RBR40 from the switch, plugged it into the RBS40 directly, and then connected the switch to the 2nd LAN port of the RBS40.

I need to emphasize again that in both cases, establishing the Ethernet Backhaul between RBR40 and RBS40 was NO problem. The problem is that as soon as the RBS40 is connected via Ethernet Backhaul (no matter with which of the 2 cabling variants), the communication between the 2 STBs is being interrupted intermittently. And again, I'm seeing this with these 2 STBs only because these are the only devices that I have that constantly transfer large amount of data. I am sure that any other LAN-connected devices that would constantly transfer large amount of data would experience the same issue.

To a certain degree I would understand if this issue would occur with the blue cabling, because in this case, all LAN traffic between devices in the bedroom and devices in the living room needs to go through the RBS40, which then might (for whatever reason) have a problem with the constant high traffic. But with the orange cabling, the RBS40 is just one more client attached to the living room switch, so the traffic between the 2 STBs is not going through it at all. But the issue also exists in that scenario.

So for whatever reason, solely the presence of the Ethernet-Backhaul'ed RBS40 causes this issue.