The issue is solved by installing xdg-desktop-portal-gnome.
xdg-desktop-portal only activates the openURI interface if
org.freedesktop.impl.portal.AppChooser is implemented.
> Oct 18 09:52:12 apollon.suse.de xdg-desktop-portal[6236]: XDP: Using gnome.portal for org.freedesktop.impl.portal.AppChooser in GNOME
> Oct 18 09:52:12 apollon.suse.de xdg-desktop-portal[6236]: XDP: providing portal org.freedesktop.portal.OpenURI