Hi @jtelserfrere,
A couple things pop in my head around this...
1 - Is the quiz assigned to "everyone"? If it's just assigned to a specific student or section of the course, others wouldn't be able to access it.
2 - Is the "available from" date before today and the "available until" date after today?
3 - It's also possible the quiz itself is loading, but the access denied message is from an embedded image/document from another course, which students can't access. if you could provide a screenshot of the error, including the URL, it would be really helpful in narrowing down the potential causes.
Look forward to hearing back from you soon!
-Chris