I tried another attempt to import the Gedcom-File. Obviously I must have made a mistake: now it worked and the ASSO-information was imported.
But to work with is very cumbersome, because the information is imported only as characters into the notes and the Indi-Nos are not resolved and filled with names or so. So you have to look for the related person in a separate step.
So far, so good, if there will be a more comfortable way to import and to handle the Asso-information, I would be thankful.
For the time being many thanks for your support. Maybe there will be more hints