This html file is located in the tour's root directory (and in the project's local \build folder). The context menu says that the html codes "are limited", so I really stripped down the html to see if it'll work:
html file vers 1 code: <p>Test text for Mazza tour.</p>
...and... html file vers 2 code: </head> <body> <p>Test text for Mazza tour.</p> </body> </html>
Both html files return the same error. Figured it's either my file placement or my html code.
As it turns out, you need to list the full and proper URL including domain in the >Hotspot Properties>Link dialogue. My mistake was that I was using a relative reference for the html file in the tour's root folder.