Same issue on Ubuntu MATE as well (as tested on hirsute daily build). On Xubuntu, when I ran `gnome-software` from terminal, I see following error (last line is when I clicked on 'Restart & Update' button).

sai@sai-Standard-PC-Q35-ICH9-2009:~$ gnome-software 
13:50:47:0180 Gs  enabled plugins: desktop-categories, fwupd, os-release, packagekit, packagekit-local, packagekit-offline, packagekit-proxy, packagekit-refine-repos, packagekit-refresh, packagekit-upgrade, packagekit-url-to-app, appstream, snap, desktop-menu-path, hardcoded-blocklist, hardcoded-popular, modalias, odrs, packagekit-refine, rewrite-resource, packagekit-history, provenance, systemd-updates, generic-updates, provenance-license, icons, key-colors, key-colors-metadata
13:50:47:0180 Gs  disabled plugins: dpkg, dummy, repos, malcontent
14:02:02:0019 Gs  CSS parse error 1:107: Using one color stop with linear-gradient() is deprecated.
14:02:03:0681 Gs  FIXME: Unknown progress handling is not yet implemented for GsProgressButton
14:02:03:0681 Gs  FIXME: Unknown progress handling is not yet implemented for GsProgressButton
14:02:03:0682 Gs  FIXME: Unknown progress handling is not yet implemented for GsProgressButton
14:02:03:0682 Gs  FIXME: Unknown progress handling is not yet implemented for GsProgressButton
14:02:50:0265 Gs  hiding category productivity featured applications: found only 0 to show, need at least 9
14:02:50:0265 Gs  not handling error failed for action get-category-apps: net/http: request canceled (Client.Timeout or context cancellation while reading body)
14:02:50:0267 Gs  hiding category audio-video featured applications: found only 0 to show, need at least 9
14:02:50:0268 Gs  not handling error failed for action get-category-apps: net/http: request canceled (Client.Timeout or context cancellation while reading body)
14:02:50:0270 Gs  adding wildcard app */*/*/*/org.gnome.Builder.desktop/* to plugin cache
14:02:50:0271 Gs  adding wildcard app */*/*/*/org.gnome.Calculator.desktop/* to plugin cache
14:02:50:0272 Gs  adding wildcard app */*/*/*/org.gnome.clocks.desktop/* to plugin cache
14:02:50:0272 Gs  adding wildcard app */*/*/*/org.gnome.Dictionary.desktop/* to plugin cache
14:02:50:0273 Gs  adding wildcard app */*/*/*/org.gnome.Documents.desktop/* to plugin cache
14:02:50:0274 Gs  adding wildcard app */*/*/*/org.gnome.Evince/* to plugin cache
14:02:50:0274 Gs  adding wildcard app */*/*/*/org.gnome.gedit.desktop/* to plugin cache
14:02:50:0275 Gs  adding wildcard app */*/*/*/org.gnome.Maps.desktop/* to plugin cache
14:02:50:0277 Gs  adding wildcard app */*/*/*/org.gnome.Weather/* to plugin cache
14:02:50:0280 Gs  not handling error failed for action get-popular: net/http: request canceled (Client.Timeout or context cancellation while reading body)
14:07:38:0573 Gs  automatically prevented from changing kind on system/package/*/generic/libgphoto2/* from generic to unknown!
14:46:35:0457 Snapd snapd_client_refresh_sync: assertion 'name != NULL' failed
14:46:35:0458 Gs  snap did not set error for gs_plugin_update
14:46:35:0458 Gs  FIXME: Unknown progress handling is not yet implemented for GsProgressButton
14:46:35:0506 Gs  Calling org.gnome.SessionManager.Reboot failed: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gnome.SessionManager was not provided by any .service files