Behavior: two days ago, about half of my desktop icons disappeared. The links still work but the picture went away. Also, when accessing the applications through the KDE Menu Editor, the small icon on the application tree is shown but the larger size icon on the right is just a gray box. Also, when attempting to re-associate the correct icon with the application (by clicking on the gray icon box), if the correct icon is not a system icon, the re-association will not work (when I click OK, the icon remains a gray box). This is very similar behavior as that described in viewtopic.php?f=289&t=124403&start=15 but that is two years old and none of the recommended remedies works now.

System:
Kubuntu 16.04
KDE Plasma Version: 5.5.5
QT Version: 5.5.1
Kernel Version: 4.13.0-43-generic
OS Type: 64-bit
Display Manager: SDDM
Icon theme: Breeze
Widget Style: Breeze
GTK 2 & 3 theme: Breeze
Fallback Theme: Various during troubleshooting but not Ubuntu-Mono-Light

Things tried:
- Clearing icon cache (sudo update-mime-database /usr/share/mime)
- Changed icon themes to something else and then back to breeze
- adding a fallback icon theme
- added a new user and the behavior was the same

Recent program upgrades:
- June 4th [libpq5:amd64 (9.5.12-0ubuntu0.16.04, 9.5.13-0ubuntu0.16.04)
- June 5th (liblouis9:amd64 (2.6.4-2ubuntu0.1, 2.6.4-2ubuntu0.2), libexempi3:amd64 (2.2.2-2, 2.2.2-2ubuntu0.1), liblouis-data:amd64 (2.6.4-2ubuntu0.1, 2.6.4-2ubuntu0.2)
- June 6th - tried to install unity-tweak-tool. it might have happened after this but am only about 50% confident about this. There is a fair likelihood that the install of the unity-tweak-tool was part of the troubleshooting on this. The failure was due to a failure in dependency (account-plugin-google).

Found the problem. LightDM had gotten installed (and I had accidentally activated it a couple days back when trying to use unity-tweak-tools). I had previously reverted back to SDDM but some artifacts must have been maintained that were messing with the icons. I did a full purge of lightdm and the 188 dependencies and after a reboot, things are back to normal.