iFrame not working

Hi There!
I have a problem with an iframe on a website of a client of mine. I have no clue, but sometimes it is just working and sometimes it is not. The iframe should display an availability calendar for accommodations.
This is the code provided by the company that handles the calendar.
—–
<script type=”text/javascript”>
var myEventMethod =
window.addEventListener ? “addEventListener” : “attachEvent”;
var myEventListener = window[myEventMethod];
var myEventMessage =
myEventMethod == “attachEvent” ? “onmessage” : “message”;
myEventListener(myEventMessage, function (e) {
if (e.data === parseInt(e.data))
document.getElementById(‘my-iframe-id’).height = (e.data +30) + “px”;
}, false);
</script>
<iframe frameborder=”0″ id=”my-iframe-id” width=”100%”
src=”https://www.vakantiehuis-frankrijk.nl/vakantiehuisfrankrijk…
“></iframe>

Deze kalender is mogelijk gemaakt door vakantiehuis-frankrijk.nl
—–
However, when visiting our website (https://domainedubonheur.nl/beschikbaarheid/), the calendar does not display. It worked a few times but then it doesn’t again. I have no clue what’s wrong with it. The developer of the frame has no clue, so maybe you guys see the problem.
Thank you