I was able to narrow done the problem:
- it occurs with V6.5 too
- it occurs when I start the application and try to open a model (any model), which I created earlier

And I found out, how to open a "old" model, but it is a bit strange:
1. Start the application
2. Save the empty model which appears ...

I can create a new model with some rooms and some standard furniture.
As long as I don't close the model, everything just works fine.

But when I close the app and reopen the model the application freezes.
So I'm not able to edit/reuse a model!

What's wrong here?
Any ideas would be extremely ...