Ok I looked into this .
please try adding this given code in your wplms-customizer.php file in wplms customizer plugin :
add_filter('wplms_get_answer_object_meta_query',function($settins){
unset($settins['meta_query']);
return $settins;
});
I guess the issue is appearing due to your wordpress hosting . We use a comment meta_query to fetch user's answer .
Please try to add this code and attempt the quiz and check it again . If it again fails then please share the user credentials who is facing this issue in private reply .
PS : As wordpress hosting is for simple site that does not use login system and content on them are mainly static but out theme is much more complex , has its own login system and registration based on buddypress .Registration may not work and would need user to create wordpress account first.