Redirect loop on login page of some subsites

Hi there,

I hope you're having a good day?

I'm experiencing a problem with some of the subsites on my Multisite (www.thinkbigsites.co.uk) where there's a redirect loop on the wp-login.php page. (Just for your info, the sites are on SSL)

When I (or any user) try to login, I get a security message displayed, which says;

"The information you have entered on this page will be sent over an insecure connection and could be read by a third party.

Are you sure you want to send this information?"

I then press the 'Continue' button, after which the page redirects back to itself.

I've played around with other subsites on the Multisite, as I knew that some of them can be logged in to without any issue. What I then discovered is that all the sites that can be logged in to without issue have domains mapped on to them and all sites that have the redirect loop don't have mapped domains. I suspect there's a clue to solving the problem in that statement but I don't know what the answer is.

Your help would be greatly appreciated.

Kindest regards,

Steve