Qt: XKEYBOARD extension not present on the X server.
Gdk-Message: 21:32:35.762: VirtualBox: Fatal IO error 2 (No such file or directory) on X server :0.
I guess this has to do with the move from Qt4 to Qt5. But I thought it's worth reporting as it may break other people's setups as well.
I too had this problem when upgrading. Did not work with my TightVNC installation, as TightVNC does not support Xkeyboard.
However, after switching to TigerVNC (which does have support), it now works fine.
EDIT: To be clear, this is not a fault with MakeMKV. But rather, with certain VNC server tools that lack the Xkeyboard extension. Any QT5 application that uses Xkeyboard would have the same problem.