by Ken Task.
Since the issue appears to be about quiz, suggest installing the Health check plugin:
https://moodle.org/plugins/tool_health
One of the items checked are quizzes. When it finishes running it presents a screen with the issues related to quizzes and provides a suggested solution if it can.
For the adhoc stuff .... go to the category of the courses having issues. At bottom of listing of courses you might see 'Pending ... blah, blah, blah'. Should be a trashcan icon. Click to remove the Pending courses.
Also, async restores/backups tips say only one per user now - meaning you can only do one of those until it completes.
Do your server error logs give any clues?
'SoS', Ken