I had this happen to me in a class this semester as well, although in my case it was with every quiz in one of my courses! I've been using Canvas for many years and this is the first time I've had this problem.
I've tried importing the quizzes in the impacted class twice now and filed a ticket with Canvas Support. (Case #12876873 in their ticketing system for anyone following this both places.) They eventually told me it was a known issue and they'd get back to me once they fixed it.
One of the other teachers at my school said he had the same problem last year in some but not all of his classes and that he just kept retrying over several days and it eventually worked, so at this point I plan to just try again tomorrow if I don't hear differently from Support by then.