The problem is that the GUIs font is not installed or not installed correctly. This is a known problem of Ubuntu which effects CarMaker. To solve this issue you have to do the following steps in the Ubuntu console (you will probably need admin rights):

1) Update package source
sudo apt-get update

2) Remove faulty package
sudo dpkg -P ttf-mscorefonts-installer

3) Reinstall package
sudo apt install ttf-mscorefonts-installer

To check if the fonts are available under Ubuntu 16.04 use the following command to navigate to the package installation path:
ls /usr/share/fonts/truetype/msttcorefonts/

The files that contain the typical font names (Arial, Arial_Black, Times_New_Roman…) should be available here.