URL in pluging causing SSL security issue

That URL does not embed anything, it’s not even a link (just an HTML comment) so it should not keep the padlock from going green. There must be other content that actually embedded from insecure sources on your site.

As the site is in maintenance mode, I cannot check it out for you but you can do this yourself by opening de developer tools in your browser (usually F12) and switching to the Network tab. Then reload the page and see the file request flowing in. Watch for the red entries or filter the list by “http:” in the search field.