We have once more a problem witch course backups. A couple of tests figured out that the problem is related to quizzes. Please follow the steps to reproduce the bug.
1. Create a question category which is associated with a course category.
2. Create a multiple choice question and insert a picture into the question using the "Image to display" field. The image should be used nowhere else as content in the course.
3. Create a quiz with this question
4. Try to make a backup of the course including user data and the quiz
Following errors will be shown up:
Linked file images/pic.jpg doesn't exist
When you exclude the quizzes from the backup, there will be no error.
I can reproduce this issue on two different moodle installations.