Thanks for you Suggestion. I created a new Foreign SMTP Domain Document from Scratch and it worked.
In fact, the Dokument wa rather old. At least 10 years. Maybe it was created on Domino 5 or 6, and Domino 9.0.1 uses Design Elements that were not avaiable in that early Versions.
I'll try the same with the configuration Document for the Address Lookup.
What confuses me is, that an --> Edit Document --> hit <F9> --> save Document doesn't update the Document the same way as a new creation.
Best Regards
Ron