@Marco Baum, sorry to hear that you have faced this issue.
However we have tried to recreate the same i.e. disabled “Course image” and “Update existing products along with” option. The new course came in without removing the image of the old course : https://snipboard.io/jwCVLq.jpg
As you are having this issue, there might be some plugin and theme conflict issue. Just to be sure disable all the plugins except WooCommerce, MooWoodle and pro and check. Please do keep us posted.