The <source> element cannot be used here (in <video> which is in <p>). This is because the <video> element can contain transparent content. In this case <video>'s allowed content is determined by its ancestor <p> element, and <p> cannot contain <source>.
This Stackoverflow Post explains that <video> is categorized as phrasing content (with links to the W3 docs), and all phrasing content is allowed inside <p>. Since <source> elements are integral children to <video> elements, it'd seem that <video> elements with embedded <source> elements are valid inside <p> elements.
Perhaps the validator is incorrect? Please advise.
Please see the "readme" file in the above ZIP file for installation instructions.
UPDATE: Should be fixed in v23.0020+. The new/updated config file disables the buggy content checking for <audio> and <video> elements and enables the fixed checking if using v23.0020 or above (which is not yet released).