Cubase sound sometimes dies when using specific I/Os in a RME multi-interface setup

Hey guys,
here’s a very weird issue that I’m trying so solve for a longer time and it happens to me across different Cubase versions (currently on Cubase Pro 12.0.3) as well as two different Windows PCs.
I’m using three RME interfaces (the RME ASIO driver allows you to use up to three interfaces of a similar type at the same time) and they work just fine. My main interface is a RME Fireface UFX which runs rock solid. Now here’s the thing: when I goto the VST-Connections and add an input/output that uses any I/O of interface two or three (not Fireface UFX), it COULD happen that the sound within Cubase just dies. I can still see all the meters jumping around but no sound is coming out of Cubase anymore. As soon as I delete that connection, the sound is back. Also resetting the ASIO driver brings the sound back in most cases but not always. And sometimes this bug just doesn’t occur and it simply works fine. No other DAW on my system has this issue - they accept any connection from any of the three interfaces without any problem.
Does anybody have an idea what could be the issue?
Thanks in advance!

Does nobody have an idea?