Lightboxed slide problem

I've run into an interesting lightboxed slide problem. Does anyone have any suggestions as to what I should be looking for in order to troubleshoot this slide transparency behaviour? 

I have the latest version of Storyline360 installed, and I am using Modern Player in Light with Menus & Controls OFF. The course is published to HTML5. 

Here's a live preview - click the Disclaimer button to recreate the behaviour. 
The How To slide is a screen shot, and it displays well. Other instances of lightboxed slides also have the same transparency issue. 

Any suggestions as to how I could fix this would be great. 

Thank you.

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