Well, I got how to enroll, the default course was 'Manual' only. Yes, for students it works fine, for instructors however not. They get the error message at the end.
What I still could not find in the Dashboard is how to add students as an instructor.
Well, when I do the same as an admin I do get the error. It occurred to me, maybe that this error only shows up for 'non students', since I have the instructor role for that course, not the student one.
Brings me to another issue... Trying to create a student, I discovered I can't find any enroll button or link... Where to enroll as a student?
I also tried as an instructor to invite the student, but the invitation only sends a link to the page without any invitation options.
I tried it on several sites and on all those sites is the same problem. Since http://develop.wordxpression.com has no link to any production environment, you might best look there. I granted you access. Just pick any course and follow it to the end and you get the message
Ok, testing it again I saw I was wrong, it is not related to the discussion but for some reason it breaks when it tries to load the course completion. I did fill out the completion for entirely however...
So still the question what setup step did I miss ;)