That error message means that the
tag requested isn't served over HTTPS. I created another tag and added an
s to the protocol, making it legitimate for serving. However, when I tested the new tag again in the HTML5 VSI, I got the error "AdError 403: Linear assets were found in the VAST ad response, but none of them matched the video player's capabilities". It turned out the creative requested is a Flash VPAID creative, and currently the HTML5 SDK doesn't support it. That explains why your tag played in the Flash SDK and didn't play in the HTML5 SDK.