Close ALL applications. All of them.
Configure Your Desktop/Application Appearance/Fonts
under "Use anti-aliasing:" Change "System Settings" to "Enabled" and then back to "System Settings".
(In other words, there will be no change, just pick any option and then pick the original option back again, don't apply the changes just yet)
This will activate the "Apply" button at the bottom of the screen.
NOW, click on "Apply".
Close the screen, open the "problem" application and see if those letters are readable.
I have been doing this for over a year on KDE.
Oddly enough, my update finally "fixed" the problem.
It seems that the update broke yours.
I never figured out why this happens.
EDIT: By the way, if it works you will probably have to do it every time you restart the desktop.