See this bug report:
https://bugs.kde.org/show_bug.cgi?id=354802
One workaround is to backup the file ~/.config/plasma-org.kde.plasma.desktop-appletsrc and use it when issue occurs again
See this bug report:
https://bugs.kde.org/show_bug.cgi?id=354802
One workaround is to backup the file ~/.config/plasma-org.kde.plasma.desktop-appletsrc and use it when issue occurs again