by Jayne Bachelor.
Hi we had the exact problem today. To overcome the issue our server admin created a copy of 2.3.1. This did create an additonal step of restoring 1.9 courses into the test moodle first however we were then able to restore from 2.3.1 to 2.4.
Not an ideal workaround but we have teachers commencing classes in a week and these course must be moved.
Hope this works for you.