Switch to the default theme and see if the issue persists. 

If it does, check if you have any wysiwig or any other editor plugin and disable it and see if the issue persists. 

Keep us updated. 

Seems it's the SUPERVIDEO (lite) plug-in that's causing the issue, I have contacted the dev. 

I need it to work as I want to embed a twitch.tv channel, but it seems this doesn't work anyway as it always shows 'connection timed out' when embeded on our main site which is HTTPS, but works on our demo site which is HTTP. 

Can you please provide twitch.tv channel embed code?

<iframe src="http://www.twitch.tv/ea/embed" frameborder="0" scrolling="no" height="378" width="620"></iframe><a href="http://www.twitch.tv/...tt_content=text_link" style="padding:2px 0px 4px; display:block; width:345px; font-weight:normal; font-size:10px;text-decoration:underline;">Watch live video from EA on www.twitch.tv</a>;

That probably happens due to the links in the iframe, they are not SSL and the SSL certificate does not support resources which are not secure

I also tried on http website and it works

Do you have a solution or anything we can try to resolve this on our HTTPS site

This is how SSL works. There's nothing we can do about it. 

The twitch.tv does not support https if you try to open this link: http://www.twitch.tv/ea/embed in https it won't load. So this is double-sided issue

Thanks Ross, i will need to have a think of what to do.

same problem Seems it's the SUPERVIDEO (lite) plug-in that's causing the issue, I have contacted the dev.