Joomla 3.8.5
PHP 7.0.27
Guru Pro 5.0.17

There seems to be a bug in the Guru system where the "next" button disappears for Windows users (Mac and iPad users are fine).

For Windows users, the next button disappears and the Prev button is the only one visible but it is an inactive button and doesn't work when clicked.

If you click the X and refresh and open up the quiz again, the Prev button will now work and the "Next" button will return on the previous lesson page page.
If you then click that, the next button will reappear on the quiz page and when you click it, it will bring up the next module heading but when you click next again to go onto the next lesson page, it takes you to the next module heading.
You can't actually access the lessons.
It's like the quiz results aren't being registered correctly for Windows users.

No problems at all with Mac or iPad users.

I have attached a picture to show the difference (this is the same page and same user account).

This has been altered to us by real users (lots of them)