Bug#823873:

[Michał Milanowski]

I have exactly the same issue on debian sid:

michal at debian:~$ gnome-software

(gnome-software:25795): Gs-WARNING **: failed to open plugin
/usr/lib/gs-plugins-9/libgs_plugin_packagekit-origin.so:
/usr/lib/gs-plugins-9/libgs_plugin_packagekit-origin.so: undefined
symbol: gs_plugin_packagekit_results_valid

(gnome-software:25795): As-WARNING **: not adding path
/home/michal/.local/share/flatpak/appstream [user-flatpak:(null)] as
not a directory

gnome-software 3.20.2-2. Tried to purge packagekit and gnome-software
and reinstall it. Still not working, there must be a bug. Please
reassign or any hint what could be reason of that? I have no any
software intstalled outside Debian Sid repo.