Does this happen in a new file too, or just this one? How about if you change the font?
What language is assigned to the text? Is hypenation enabled?
Do you really mean that if you continue to type you go from one visible line and overset to three visible lines and no overset? Some screen shots of the overset frame and the Story Editor, both with non-printing characters displayed would be helpful. Use the camera icon on the web page like this:
First of all, thanks a lot for your help. Let's see...
1. This happens in all files
2. It happens will all fonts and languages
3. As you can see for yourself, hyphenation is in fact enabled, and the most weird thing is that it occurs with most characters, but not all. We have seen that this problem does not occur when we use K and W, for instance.
If you need anything else please do let me know.
Once again, thanks a lot!
I'm about to run out for the day, but one thing I notice is the language is set for German. Is that the language you are using? What happens if you change the lanuage (don't use none, though, as that won't hyphenate)? Are youpatched to the latest update for your version? If so, you might also want to try trashing the prefs. See Replace Your Preferences
Can you post a screenshot of your Keep Options?
I don't think this can be a keeps problem since it's all one paragraph, and there is space in the frame, but that's an interesting thought.