Thank you very much for your quick reply.
The website in question is on production, so I imported your demo quiz to one of my other websites on the same multisite network.
The direct link to the quiz works just fine, but when I implement the quiz in a blog post, I doesn't work.
I tried changing the theme to the default Twenty Twenty, and it did work, so I guess there is something in my theme's post template that prevents the qquiz from working properly.
If you could look into it and tell me what it is, I could ask my developer to fix it.
Thank you very much,