Touchscreen stops working a few seconds after login to Gnome/Ubuntu session (Ubuntu 20.04)

Solution 1:

Looks like it's this bug: https://bugs.launchpad.net/bugs/1876991

Marking it as affecting you too will increase chances it will be noticed. Also you can subscribe for updates there.

If your touchpad is set to be disabled with mouse you can try these settings:

GNOME: gsettings set org.gnome.desktop.peripherals.touchpad send-events enabled command, Details: How do I disable touchpad when using a mouse

KDE: System Settings > Input Devices > Touchpad > Enable/Disable > Ignored Devices > Add your "mouse" touchpad/touchscreen entry, should be already filled in as it's "connected".