by Ken Task.
Without knowing what's in the course ... knowing only that if you do backups selecting say section 1 -> 3 and no more, that works ... how about a different approach that might discover where the issue might lie?
Do a GUI backup and this time de-select all quizzes in the course ... when it comes to naming the backup insert -noquiz- in it's name. If that works, make another backup and this time de-select all other resources in the course *except* quizzes and in the name of the backup insert '-quizonly-'.
Do both fail? Does one succeed? Which?
If one had debugging turned on while attempting above might get some more info.
'SoS', Ken