It got fixed!
export QT_QPA_PLATFORM=xcb
did the trick. But I think Ubuntu noticed this automatically, because I eventually got at the same 'hanging' stage as the first time. After running the calibration a pop up comes up saying 'unknown - 'cover image' ready'. There is nothing like a commit button or similar.
I was working on a pretty small screen so it could be that I've missed info from the terminal. I eventually just pressed space randomly and that basically was the key, from there on I could accept every step/image with hitting 'space' and got eventually the green finish screen.
Thanks again. If needed I could send some photographs from the process.