volgens Debbie Unterseher.
To get some of my courses to upload, I had to create a new empty course on the old Moodle 1.9 server, and Import the old course to the new empty course (still on the old server).
Then I backed up the newly created course on the old server and downloaded the .mbz file (On a side note, the file size dramatically decreased. It was a 1.4 GB file that shrunk to a .6 GB file size!) I then used the Restore option on the new Moodle 2.3 server to pull in the course, and it worked!
Good luck!