I am having the same issue
My course is very large - I have the course in 1.9 and I am trying to restore it to 2.4
I have tried - backup full course.
Restore full course - In the item list - about 3/4 way down the icon with the triangle/lock appears beside each item (locked by dependencies)
I have tried to only back up the non-quiz items and restoring that file , but I get the same thing.
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What does the locked by dependencies actually mean?
Thanks
Alice