I've just tried again, the same as yesterday.
It is a big file, yes, 4'280 KB, but that should not be the problem. I remember i had problems with another course, but there i recieved a message, like "too many object" or similar. If it's the size of the file, or the number of objects, I think it should give out the corresponding message. That would help to adjust the course.
@KeithC, would be happy, if you do it for me. I do trust you ... Thanks for the offer.