thank you for follow up.
url moodle.waldorf..... was the firstly updated version to 3.9 which did not work
The new url is moodle3.waldorf.... which is now a weekly built version 3.9.1+
(The different urls have been just for safety reasons to keep the database and moodle-data directoriy for the different versions)
With this version Moodle 3.9.1+ (Build: 20200807) I got it running again and the Activity Chooser is also working again!
So this is solved for me,
thank you all,
P.S.: for all with similar "invalid user" - error, which I mentioned above: The reason for this error message was the deleted "Guest User"!
Discussed here: https://tracker.moodle.org/browse/MDL-69076
(In my moodle database was for username=guest => deleted=1 which I had to correct to deleted=0)