Comment # 1 on bug 965653 from Wolfgang Bauer

That kactivitymanagerd crash is likely unrelated and can be ignored.
It's a bug in Qt5 and has been workarounded in later versions of kactivities5,
but shouldn't cause real problems.

The sddm-greeter segfault looks more serious, but would also not cause the
desktop session going unresonsive. Maybe it's even just caused by you killing
the X session.

More likely IMHO is a graphics driver problem, or something taxed the CPU
and/or caused heavy file io or swapping.

Might have been baloo, the file indexer, disabling Desktop search should
circumvent the problem then.
Or it might also have been some other random application going crazy and
slowing down your system.

As I understand it, the actual problem is "fixed" now, right?

If not, a look at "top" when it happens might provide a clue...