Hello,
Ubuntu 18.04.1 LTS, QT 5.9.5 , KDE Frameworks 5.44 , Plasma 5.12.6

The pop-up notifications that show the arrival of new mails or printing tasks are not formated well. Some fonts are only black squares.
Also in the taskbar the labeling of the windows are similar.

Is there a way to fix this ?
Thank you

Seems to be related to KDE system wide fonts. Did you changed it anyway? Or did you remove some fonts?

I tried to change them from Noto Sans to other : Gentium , Monospace, .
It is a little bit better but not as it should be.
I did not remove any fonts.
Thx