"Updates" shows no updates then "Could not download packages" before updating

I boot into Kubuntu every couple of weeks, so each time I know there are lots of updated packages to install. Twice the process has said "no updates", displayed an error, then eventually gone ahead to update anyway. Here's the sequence.

1. Boot Kubuntu after a few weeks.
2. Click to reveal hidden icons, click the Updates icon in the panel.
3. The notification window switches to show "Updates | System is up to date"
4. I know this is wrong, so right-click the Updates icon and choose "See Updates..."
5. The Plasma Discover window appears "The software on this computer is up to date."
6. But then an error dialog appears:
     Failed to App...es — Discover
     Could not download packages
   clicking [Details>>] in the dialog does nothing; see attached screenshot.
7. And then Discover displays a bunch of packages that require update!

The first time this happened I clicked the [Update] button in Plasma Discover and Update proceeded.
The second time the Discover window showed an incomplete progress bar and no update button; I had to close it, reopen, and then I could update.

I suspect that if I let the system "settle down" and check for updates in the background then the panel icon would tell me I have updates and all would be fine. But when I force "See Updates..." right after booting it goes wrong.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: plasma-discover 5.6.2-1ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-22.39-generic 4.4.8
Uname: Linux 4.4.0-22-generic x86_64
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
CurrentDesktop: KDE
Date: Wed May 11 12:01:19 2016
ExecutablePath: /usr/bin/plasma-discover
ProcEnviron:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/zsh
SourcePackage: plasma-discover
UpgradeStatus: Upgraded to xenial on 2016-04-28 (13 days ago)
XsessionErrors:
 (process:4686): GLib-CRITICAL **: g_path_get_basename: assertion 'file_name != NULL' failed
 (process:5146): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed