Tom, I must wait now at least 3 days, until I can see the Google Crawler results following the 27th, the day I created the robots.txt
In the meantime I checked the Web and found a few similar problems. I dont want to spam links, just to share what I have found. If you dont want something like this here, let me know.
The problem seems to be old and the cause was never really found:
The problem seems to be old ….
Someone says Cookies are the Cause
A similar situation – solved by the Robots Fix I try now – cause unknown
One Topic in Google Forums with a similar problem – cause unknown