Bug#567216: closed by Sune Vuorela <email@example.com> (plasma-widgets-workspace: system monitor widgets empty)
I installed all recommends recursively, including ksysguardd, that wasn't the
problem. But I solved it in the meantime. I forced a reinstall of those
dependency libraries, after that, everything worked properly again. I have no
idea though why some library was faulty after the upgrade.
As some other reporter already noted though, there should be some error
message indicating when some library etc is missing. Especially as
installation is quite error prone and people are stumbling in the dark this
way. For a newbie it might be really frustrating. Again: all recommends were
installed on system!