Hi Dorothea
Glad to hear one of your avenues is working.
Let me be clear - Ubuntu 18.04 to lxplus works but Mac to lxplus does not. So it looks like something is strange about the Mac. However, when you are connecting remotely to a G4 app using “ssh -Y” you are not using Qt on the Mac. You are using Qt on lxplus and using the X windows server/client mechanism to get a picture on the Mac. So perhaps something strange about XQuartz on Mac. What versions of Mac OS and XQuartz are you using?
I’m not in a position to try this myself just now, but I hope you can make progress with your Ubuntu machine.
John