I am not sure if I can shed light on this or not, I am also sure that this is just a workaround and not the fix for the underlying issue? When we first started, all of our courses went from 1.9 to 2.7 without any issues. Then after running for a few months, we tried to restore a couple of courses again, and we received the error above. After testing some courses, we found that if we restored the courses without the quizzes they went in just fine. So what we have been doing is exporting all the quizzes from a course in one instance of Moodle and then importing them into our current running version. For now, this is how we are getting our courses back in. I am not sure why it worked for so long and then decided that it didn't? So if anyone finds a way to fix the error, I would love to know.
Thanks, Angel