I missed the license dropdown. That caused the error. I had to delete the draft and start over. Then it went through without issues.
WORKAROUND: Do not fill in all the fields then save. Instead, fill in just the name then save the draft. Then open the draft again and fill in the remaining fields such as description, screenshot and mendix version. For some reason this does not trigger the error message.
I also received the error four days ago, having fully filled all fields and upon clicking save draft. My workaround was to create a github-project and a release for it, including the package, and using the “Select from GitHub” instead of “Manual upload”.
@Marcel thanks for the comment. I checked my screenshots and noticed that the license field is missing from the form completely. Very strange. I will add this info to the support ticket, maybe it helps in solving the issue.