Test not submitted

We are experiencing strange issues with the test in our title. On the page with the last question, a button is added with an action to submit the test score. In the past, users could always click this button and they were instantly directed to a page telling the user whether he/she succeeded the test.For some reason, clicking this button now does not always move the course forward. On my computer it works sometimes. On a colleague's computer nothing happens at all.The browser is not frozen. You can still move backwards, but moving forward is not possible anymore.Generally speaking, nothing changed to the overall setup of the title. That's why we do not understand how this can happen. The output we use is an HTML output. All help is welcome. Thank you!