Comment # 25 on bug 1013550 from Wolfgang Bauer

(In reply to Michael Hirmke from comment #24)
> Your kio packages have version 5.26.0-5.1, while my installed default
> openSUSE versions already had 5.26.0-6.2. So I had to downgrade those two
> packages - was that really, what you wanted me to do?

Yes.
That's just the build number, and it's lower because I branched from
KDE:Frameworks5:LTS and not the standard Update repo.
It is not really a "downgrade", actually it contains two additional fixes...

> It didn't help, though:
> I installed both packages and rebootet the system, but the error remained
> unchanged 8-(

Well, what I suspect is that if a notification that triggers the freeze is
still in the history, it will trigger the freeze again whenever a new
notification appears.
Would explain why it works fine for you for a while and then suddenly all
notifications trigger the problem, even after a reboot.

That patch only helps with new notifications though, that are emitted by kio's
file operations.
It probably won't help if the problem exists already.

So you'd need to remove that history for a proper test IMHO.

I'm not sure currently where this history is saved though.
You say that removing .config/plasmarc helps, so try that (with the new
packages installed).
But I would actually rather suspect
.config/plasma-org.kde.plasma.desktop-appletsrc...

The safest test would be on a new user account of course.