Sorry for the delayed response. That error is returned if the format of the HTML content doesn't contain the minimum required fields for valid HTML. Is there any chance you can post a copy of your JSON you're sending over or the HTML content you're sending us so we can take a look? It could be as simple as something missing in the HTML or something that you are using in your HTML that we are handling incorrectly.
Dave Berard Senior Product Manager, Constant Contact