I have my moodle instance running on Linux, and it's being used by the company that I am working for. We have created a handful of quizzes for a particular course. Some registered users for the course are unable to submit a quiz, and instead they get the error "There is no quiz with id 0." This happens to about 1/4 of the users, and they have NEVER been able to submit the quiz they re-try over and over. The other 3/4 of users never have a problem and can submit successully every time. Also, I have logged in as those users (as a student role) who are unable to submit, and I have never been able to replicate their problem. Submitting the quiz as myself has never resulted in a problem either....
Does anyone have any ideas why this might be happening?