WordPress Multisite Strange Redirect on Primary Site

I am using a plugin which allows for multiple TLD’s with WordPress multi-site. (WordPress MU Domain Mapping) No idea if this is a conflicting issue or not. Doubtfut, but thought I would throw it out there.

Every now and then, when I visit the primary website which multi-site was installed on, I get redirected to a page: http://www.primarydomain.com/wp-signup.php?new=primarydomain.com

Displaying the following text in a yellow box:

Greetings Site Administrator! You are currently allowing “none”
registrations. To change or disable registration go to your Options
page.

and a string of text beneath that:

Registration has been disabled.

Although, I already added define('NOBLOGREDIRECT', 'http://www.primarydomain.com/'); to wp-config.php

Why is this happening?

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