The "Software" application (installed from the Software Boutique) that allows the installation of deb, flatpak and snap applications is not showing any applications (except on the home page). All the categories when selected are blank.
I tried this on two different Ubuntu MATE installs (one is fresh).
The error output is :
00:37:12:0817 GLib-GIO g_input_stream_read: assertion 'G_IS_INPUT_STREAM (stream)' failed
00:37:13:0325 Gtk Could not load a pixbuf from icon theme.
This may indicate that pixbuf loaders or the mime database could not be found.
That fixed the issue on the older installation but did not fix the issue on the fresh install. Comparing the two installs only the older (upgraded) system was displaying the error.
The fresh install system just doesn't display anything without any warning or error. So I'm looking for another issue on that machine.