Registration link on sub-site take user to main site's registration page

I'm having a problem with setting up Multisite, which I've not had in the past.

When I set up a sub-site and click to create a user account on that sub-site, it takes the user to the main site's registration page.

Here is the sub-site:

If you click on the registration link, you'll see it takes you to the main site.

I originally had this problem using sub-domains, then I reinstalled WP & reinstalled using sub-directories, both with the same result.

I have clicked on my Permalinks after setting up Multisite, just in case.