The reason for this warning is because ResolveUrl("Load.ashx?type=style&file=SyntaxHighlighter.css") will always resolve to http. If your site is running in https it will bring up the warning (to get the warning make sure your IE browser has "Display mixed content" setting disabled).

 

To get more info on why this issue is happening in general (not specific to CuteEditor) see  

CuteEditor team - please fix this bug: you would need to update this line to support resolving to HTTPS or come up with another solution to load this css file.  The reason why you can't reproduce this is probably because you don't have HTTPS setup locally.

 

Thanks,

Oleg