Hello,
When I start the computer I often get a "default" workspace. Here is what I loose :
- Taskbar settings
- Bookmarks in the start menu
Probably other things I did not notice. (I did not try to add widget, change the wallpaper, but I think this would also get wiped out)
This does not happen every time. I just tried to boot, make some changes, reboot, and it was ok. But if I run KDE for a longer time, I often loose my settings.

What I never loose :
- Document history in start menu
- Startup programs

Plasma 5.5.5
QT 5.5.1
Kubuntu 16.04.2

Thanks