KDE Font problem in Ubuntu 20.04

[Christoph Pleger]

Hello,

a few days ago, I installed Ubuntu 20.04 from the default installation DVD. 
Afterwards, I additionally installed packages plasma-workspace, sddm and kde-
full. Then I rebooted, so that the sddm login screen was shown, with breeze as 
theme. Unfortunately, I found that the login screen had two problems:

1. Text is either not displayed at all or in a yellow with a font
that I would say the test is illegible, but possibly some words can be 
guessed. 

2. The action buttons (suspend, hibernate, etc.) are not shown, though they 
are defined in /usr/share/sddm/themes/breeze/Main.qml.

Then, I started a Plasma session (I was glad that with the bad font I found 
the correct menu entry) - in that session, I could not find displayed text 
anywhere - in the menu, the application icons were shown, but the application 
names were missing. 

Yesterday, I performed a new installation, this time with the Kubuntu 
Installation DVD. But though SDDM is the default graphical login manager of 
Kubuntu, the situation is even worse there - on the login screen, even the 
arrow to click on for opening the session selection list (for choosing Plasma, 
or Gnome, etc.) is not shown.

What's wrong? On a machine with Debian 10, which is probably quite similar to 
Ubuntu 20.04,  I initially had the same problems with SDMM, but after 
installing additional packages the problems were solved - but I guess that 
these were about 2000  additional packages (I had copied the package list from 
another computer where the problems did not occur) and I have no idea which 
package(s) of these provided the solution. 

Best wishes
  Christoph