The issue is down to the javascript right after the button/input element for start quiz is incorrect – in my case, it was surrounded by paragraph tags
<p> <script type="text/javascript">
window.wpProQuizInitList = window.wpProQuizInitList || [];</p>
<p> window.wpProQuizInitList.push({
id: '#wpProQuiz_1',
init: {
quizId: 1,
mode: 3,
globalPoints: 24,
timelimit: 1200,
resultsGrade: [95,80,62.5,37.5,25,0],
bo: 1024,
qpp: 0,
catPoints: [24],
formPos: 0,
lbn: "Finish quiz",
json: {"1":{"type":"multiple","id":1,"catId":0,"points":1,"correct":[0,0,1,0]},"2":{"type":"multiple","id":2,"catId":0,"points":1,"correct":[0,0,1,0]},"3":{"type":"multiple","id":3,"catId":0,"points":1,"correct":[0,0,1,0]},"4":{"type":"multiple","id":4,"catId":0,"points":1,"correct":[0,1,0,0]},"5":{"type":"multiple","id":5,"catId":0,"points":1,"correct":[0,1,0,0]},"6":{"type":"multiple","id":6,"catId":0,"points":1,"correct":[0,1,0,0]},"7":{"type":"multiple","id":7,"catId":0,"points":1,"correct":[0,0,0,1]},"8":{"type":"multiple","id":8,"catId":0,"points":1,"correct":[1,0,0,0]},"9":{"type":"multiple","id":9,"catId":0,"points":1,"correct":[0,0,1,0]},"10":{"type":"multiple","id":10,"catId":0,"points":1,"correct":[1,0,0,0]},"11":{"type":"multiple","id":11,"catId":0,"points":1,"correct":[0,1,0,0]},"12":{"type":"multiple","id":12,"catId":0,"points":1,"correct":[0,1,0,0]},"13":{"type":"multiple","id":13,"catId":0,"points":1,"correct":[1,0,0,0]},"14":{"type":"multiple","id":14,"catId":0,"points":1,"correct":[0,0,1,0]},"15":{"type":"multiple","id":15,"catId":0,"points":1,"correct":[0,1,0,0]},"16":{"type":"multiple","id":16,"catId":0,"points":1,"correct":[0,1,0,0]},"17":{"type":"multiple","id":17,"catId":0,"points":1,"correct":[0,0,0,1]},"18":{"type":"multiple","id":18,"catId":0,"points":1,"correct":[0,0,0,1]},"19":{"type":"multiple","id":19,"catId":0,"points":1,"correct":[0,1,0,0]},"20":{"type":"multiple","id":20,"catId":0,"points":1,"correct":[0,1,0,0]},"21":{"type":"multiple","id":21,"catId":0,"points":1,"correct":[0,0,0,1]},"22":{"type":"multiple","id":22,"catId":0,"points":1,"correct":[1,0,0,0]},"23":{"type":"multiple","id":23,"catId":0,"points":1,"correct":[0,1,0,0]},"24":{"type":"multiple","id":24,"catId":0,"points":1,"correct":[1,0,0,0]}} }
});
</script></p>
Will advise on a fix if I find one.