max_input_vars setting controls only how many fields can be posted at once in a form. A quiz of even 200 students would not blow the default. If the other suggestions don't work (like overridden), I'd run a regrade on the quiz which can often fill in those gaps that occur for unknown reasons.
Thank you for your answer.
Can you explain me how to run the regrade on the quiz?
It is one test divided into 4 "mini-quizzes". The final score is calculated on the base of the the 4 "mini-quizzes" scores. Do I need to run the regrade on the 4 "mini-quizzes"? I guess yes because the scores of all the "mini-quizzes" do not appear in the grader report and consequently the final score is missing too.
Thanks Bob. So I understand that I need to enter the grader report, enter each quiz and where I need to click "regrade all" or "dry run a full regrade?
Dry run just shows you what's going to happen which might be nothing, according to dry run. To get the grades into the gradebook (if this works) you need to run a full re-grade.