I thought I could select multiple textframes in InDesignand make the same changes for the font in all of them at once by opening the font window, but that is blanc. (see screenshot)
What am I doing wrong?
It should be possible. Try to reset preferences: quit InDesign and relaunch it while holding down the Shift, Ctrl and Alt keys [Windows], Shift, Ctrl, Alt and Cmd keys [Mac] until you see a warning message asking for confirmation.
GREAT! it worked. Thanks a lot for your help!
Font size is NOT a text frame property. You should work with paragraph styles and change the font size there in charcter properties.
sorry for using the wrong terms! Yes, I did the detour by using paragraph styles, but this led to a long series of formats. After the reset everything is fine now. Thanks for your support!