because I was having similar issues to those noted in #578, I upgraded packages as per the advice in that issue : mark@z800:~$ pip3 install --upgrade PyQt5 PyQtWebEngine Defaulting to user ...
We have PyQt5 app that uses the PyQtWebEngine (embedded Chromium browser) component, which we freeze with cx_Freeze from a venv virtual environment. We successfully frozen this application in the past ...