Very weird behavior when connecting RBS50 via ethernet

[matg22s]

Hey everyone,

I have the following setup
/-> (1st ethernet port) RBS50
Internet - RBR50
\-> (2nd ethernet port) HOST A

While RBS50 is connected via ethernet to RBR50 - I can't access (e.g. ping, etc) HOST A - although HOST A has nothing to do with RBS50!
If I unplug RBS50, it then connects via 5g backhaul to RBR50, and I can access (ping) HOST A as expected
Plug RBS50 again - and access to HOST A is lost

I'm running V9.2.5.2.23SF-HW but stock firmware also had the same issue.
I tried looking at routes, ifconfig, dmesg, everything - but didn't find a clue

All help is welcome!