I upgraded my Ubuntu installation, and Python 3 was upgraded from 3.7 to 3.8. After I installed it with pip3 install leo, Leo tried to start but failed. The error message involved an xcb plugin for PyQt5.
It seems that other people had the same problem with other apps. I found a solution at https://forum.qt.io/topic/93247/qt-qpa-plugin-could-not-load-the-qt-platform-plugin-xcb-in-even-though-it-was-found/3:sudo apt-get install libxcb-xinerama0Somehow, this fixed or reinstalled the xcb plugin. After this, Leo loaded normally. BTW, the PyQt5 version is 5.15.0.