Comment 9 for bug 1869561

Sharing some findings after having poked a bit to the issue, the problems come from the /usr/lib/libreoffice/share/registry/main.xcd configuration, copying that file from the Debian build makes the documentation open in the default webbrowser and work.

The Ubuntu version is having no mention of DesktopBackend which seems relevant

The build log has those warnings which don't exist in Debian or Bionic
'...
no obvious way to instantiate implementation "com.sun.star.comp.configuration.Update"
no obvious way to instantiate implementation "com.sun.star.comp.configuration.backend.DesktopBackend"
no obvious way to instantiate implementation "com.sun.star.comp.configuration.backend.KF5Backend"
...'

They seem to come from tests and probably not the direct but could hint on an issue, if the backend fails to load then it could end up missing from the configuration

There is nothing obvious in the packaging diff between Debian and Ubuntu though...

Including the Debian file so it's easier for other to check the differences / test if it works for them as well by copying it over the system one