Sorry that I only posted the quick and dirty solution yesterday, it was already very late for me 
If you know which widget is the cause, then it is enough to delete the relevant part of the widget in the plasma-org.kde.plasma.desktop-appletsrc file. This also works in booted Linux (and if not, then via live iso). For example, using the memory usage widget:
If you don’t know what is causing the issues, then simply delete the entire file - as soon as Plasma is restarted, the file will be recreated with the default settings. This will of course delete the user settings and you will have to set up your desktop again.
If you still have the backup of the file, then open it with an editor, delete the lines with the faulty widget and copy the file back to /home/username/.config/ - this will save you having to set up the desktop again.