Offline updates are failing with no error message

As of yesterday, every attempt to do an offline update fails. I found this in /var/lib/PackageKit/offline-update-completed:

[PackageKit Offline Update Results]
Success=false
ErrorCode=failed-initialization
ErrorDetails=Failed to find gnome-software;3.32.1-3.fc30;x86_64;updates

Strangely, GNOME Software never presents this error message. It used to inform the user that the update has failed using a desktop notification. That's not happening anymore; the user has no indication that anything went wrong, other than the fact that the update process finished at 10% rather than the usual 100%.

Kalev requested a gnome-software --verbose log. This is me opening gnome-software after the failed update and clicking the Download button:

log

Kalev requested a list of files in /var/cache/PackageKit; this is what I have there (after that download has completed):

files