PaTER68 By chance, is your Ubuntu 23.04 using Wayland as it's graphics configuration?
As stated in the 3.0.2 release blog:
"OpenLP at present does not behave well under Wayland so the recommendation is to run under X11. If you can't run in X11 (or prefer to run it in XWayland), you should start it with QT_QPA_PLATFORM environment variable set to xcb, although the Main View from the Web Remote will not work."
If not, then not sure... as a church, we use Mint more than Ubuntu.