I think it relates to the diskimage URL being http and not https.
e.g.
The error I see is
When I manually change the discimage part of the URL to https it then works as expected.An error occurred - sorry!
While initialising:
Error: A network error occurred loading http://bbcmicro.co.uk//gameimg/discs/74 ... oggerP.ssd
The browser console, has this error
Which confirms that it's the browser blocking HTTP content when the main page is served by HTTPSBlocked loading mixed active content "http://bbcmicro.co.uk//gameimg/discs/74 ... oggerP.ssd"