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
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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.
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.