It looks like the pacman backend is out of date, I had a quick look at the code but don't know enough about C++ or packagekit to debug the issue.
i'm using Discover (I couldn't get gnome-packagekit to display anything meaningful on kde) but it crashes when trying to perform any installation/update. It correctly displays all the installed packages and lists packages that need updating it crashes whenever an installation is made. It looks like this issue is part of packagekit and not Discover.
I can provide crash logs, but simply trying to install updates or install a package will replicate the problems.
We moved the upstream bugtracker to GitHub a long time ago. If this issue still affects you please re-create the issue here: https://github.com/hughsie/PackageKit/issues
Sorry for the impersonal message, and fingers crossed your issue no longer happens. Thanks.