Hi Gert,
This chart for example.
https://cloud.highcharts.com/show/aseqyg
<iframe class="highcharts-iframe" src="//cloud.highcharts.com/embed/aseqyg" style="border: 0; width: 100%; height: 500px"></iframe>
Seems to work fine, but when I use the embed link, it doesn't seem to work.
Another thing that points me to this is that the image in the overview for all the charts from this user is a default placeholder and not a preview of the chart like it is for other charts.
Also the hotlink images seem to give the default placeholder:
//cloud.highcharts.com/images/aseqyg/400.png
//cloud.highcharts.com/images/aseqyg/400.jpg
//cloud.highcharts.com/images/aseqyg/400.pdf
//cloud.highcharts.com/images/aseqyg/400.svg