I exported the project to html 5 and opened it on open server. I also have reg.php and connect.php files there. But the game doesn't want to send an ajax request to it. Regular registration page
See "Making AJAX requests cross-domain or in preview" at
construct.net/en/make-games/manuals/construct-3/plugin-reference/ajax
Some things also don't work without a shttp server, which is pretty much required on all online websites now. You should be able to configure it on your own local server.
Develop games in your browser. Powerful, performant & highly capable.
And by the way, I don't understand. Why is there a preview version. I exported the game to html and transferred it to the server. I haven't tried it in preview yet.