After upgrade to 5.9. the font settings of some qt-apps (virtualbox, keepassx, clementine) discontinued. Font changes in systemsettings don't apply anymore!
Changes through qt4-qtconfig can only be saved for exactly one session, they do not survive a reboot.
The workarounds here in the bugreport https://bugs.launchpad.net/ubuntu/+source/qt4-x11/+bug/298781 are not quite appropriate, because I can actually change fonts within a session, but as i said just within one session.
~ / .config / Trolltech.conf seems to be the related conf, and it keeps the changes i made even after a reboot, but they are actually not applied in a new session!
Is Trolltech.conf not being read, is it ignored or otherwise hindered at startup

maybe qtbase-abi-5-5-1 is part of the problem?
For i want to install qtcurve but this can not be done due to broken dependencies with qtbase-abi-5-5-1

The file ~/.config/Trolltech.conf is indeed updated correctly for the settings in the [Qt] section at top. However on login, another section [qt] is added at the bottom with settings that override the [Qt] section .

I solved this by making ~/.config/Trolltech.conf readonly.

If I want to use the qt4 qtconfig utility, I make it writeable, make the change, then make it readonly again.

That way my changes are persisted across sessions.

@ cartland

thanks a lot for your hint! Its simple and quite practicable.
Nevertheless are there fonts out in www which don't work in no way.