The issue comes from the Wp-Pro-Quiz plugin. Please follow the steps below toto get it resolved:

1. Open the following file:

wp-pro-quiz/lib/view/WpProQuiz_View_FrontQuiz.php

2. Find the following lines, and replace with the following ones:

2.1. line 623

apply_filters('comment_text', $this->quiz->getText()) 

2.2 replace with the following one:

$this->quiz->getText()

2.1. line 761

apply_filters('comment_text', $resultText) 

2.2 replace with the following one:

$resultText

2.1. line 871

apply_filters('comment_text', $question->getQuestion())

2.2. replace with the following one:

$question->getQuestion()

2.1. line 985

apply_filters('comment_text',$clozeData)

replace with the following one:

$clozeData

2.1. line 995

apply_filters('comment_text',$assessmentData)

2.2 replace with the following one:

$assessmentData

2.1. line 1032

apply_filters('comment_text',$question->getCorrectMsg())

2.2 replace with the following one:

$question->getCorrectMsg()

2.1. line 1060

apply_filters('comment_text',$question->getCorrectMsg())

replace with the following one:

$question->getCorrectMsg()

2.1. line 1062

apply_filters('comment_text',$question->getIncorrectMsg())

2.2.replace with the following one:

$question->getIncorrectMsg()

2.1. line 1081

apply_filters('comment_text', $question->getTipMsg())

2.2. replace with the following one:

$question->getTipMsg()

Please don't forget to delete all caches and press CTRL+F5(twice) on the frontend before checking.

<p>
<?php echo do_shortcode(apply_filters('comment_text', $this->quiz->getText())); ?>
</p>

I'll keep on looking into it to make sure all's good.

Sure please keep us informed. 

Thank you, too, for the couple of useful plugins your developers have installed on my site (Query Monitor and AceIDE)! 

Yes the plugins are very useful. You can keep those plugins. 

The hook is already removed. Please test it and let us know if you see any issues. 

If you do, and it causes other conflicts, will you be kind enough to restore the original at my request ? If so, yes, please go ahead and delete it, and we'll see how it goes.

The developers have checked the codes of the plugin. The only way to solve the issue in the plugin code is deleting that hook. However, we're not sure if it may not cause other conflicts. So please let us know if you confirm to delete that hook.