Hi, thanks for your reply.
I have followed all driver-installation procedures carefully. Before that my Focusrite was not recognised by Logic at all, so there was no option to do it otherwise. After their instructions it works well.
If no one here has this kind of I/O-issue, I will ask Focusrite off course. There GUI of the control-software shows several 'wrong' lines in the graphic figures, like it's not completely finished yet. May be that is the issue... Hope so.