Comment # 27 on bug 1013550 from Wolfgang Bauer

(In reply to Michael Hirmke from comment #26)
> So the only cur at the moment really seems not use Air openSUSE as Desktop
> theme.

Well, I was using the "Air openSUSE" desktop theme since then on my (radeon)
system without a single problem.

But yesterday, I tried on a different (intel) system, and immediately
encountered the same problem, every desktop notification froze Plasma.
Strange enough, I was not able to reproduce it on a different user account, so
I investigated the difference in the settings.

It was related to the configured font!

"Sans Serif 9" (which maps to "Arial" here) causes the freeze for me, with the
default "Noto Sans 10" (or even "Sans Serif 10") everything works fine.

Though I suspect it would also depend on other factors, like the exact font
metrics or the DPI.

The theme probably only comes into play because different themes may have
different margins (which affects the text layout/rendering of course).

So please try whether changing the font helps in your case as well (if only to
confirm that you actually experience the same issue).

I vaguely remember some upstream bug reports about this (in Qt)...