One other item that comes to mind is that you may have to install some of the xcb packages. If calibre starts with an error about loading xcb, see
Calibre 5.0.1 Could not load the Qt platform plugin "xcb" FIXED for some help.
Edit: Going back to my notes for Ubuntu 20.04, I needed to run:
Code:
sudo apt-get install libxcb-xinerama0
to correct the startup error.