HTTPS problem when using only registered users

Hi,
My quiz (plugin version is 0.33) is configured for registered users of the site. So I need to login to answer the quiz. My server has a certificate installed and my wordpress is using https for login and admin pages.
So when I try to login, I do it in a https url. After that, I continue to navigate in the site with http. The point is: when I try to start the quiz, I always get the following message: “You must sign in or sign up to start the quiz.” On the other hand, if I access the quiz url with https, I can answer it normally. Can you please check what is going on?

https://wordpress.org/plugins/wp-pro-quiz/