Comment 28 for bug 1563155

None of the above worked for me. Instead, there were several plugins that gave the following error:

Failed to parse /usr/share/app-info/xmls/org.freedesktop.fwupd.xml file: cannot process file of type application/x-go+xml

I later got a similar error at /var/cache/app-info/xmls/fwupd.xml

For the time being, I removed the fwupd.xml files (after making a backup) and all works fine again. When I check the mimetype it shows application/xml; I'm not sure where the application/x-go+xml is coming from.