Ok, thank you. Yes, I can uninstall/reinstall LO.
I usually do not use OpenLP to do presentations, but in our Church, I have setup our data folder on our owncloud installation, so that we can share the data between different uses and prepare a service from home...
22 April === success, at last!
I managed to reinstall OpenLP – directly from Ubuntu software center – at last.
But only after having suppressed or deactivated several sources – including OpenLP (and reactivate others), an run an update/ugrade. LibreOffice did update to version 7.1.2.2 10(Build:2), coming directly from Ubuntu sources (libreoffice-ubuntu-ppa-bionic.list).
OpenLP is now version 2.4.6 and function normally,
As for python3, I have version 3.8.5