"Quiz is loading"

I have an issue with the quiz that is bothering me right now :

I made a custom field which has to be filled and the beginning of the quiz, it was working so far, but now, when the field is left empty and someone tries to start the quiz, instead of the “you must fill this field” message i have “quiz is loading” and nothing else. It works fine when the custom field is not empty.
I would also like to know where i can edit the “quiz is loading” message. Any hint would be appreciated.

Thanks.

PS: quiz is hosted locally.

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