Ubuntu users, if you're having trouble getting updates (and your computer is running hot because appstreamcli is eating all your CPU), there's a pretty easy solution: disable xenial-backports and run the updater.
Open Software & Updates, go to the Updates tab, and deselect "Unsupported updates (xenial-backports)".
Click the close button and refresh the software list when prompted.
Next, launch the Software Updater to install updates.
After the updates are finished installing, you can go back to Software & Updates and reselect "Unsupported updates (xenial-backports)". Future updates should work after this.