Formatting Issue w/ Private Messaging

Hi There,

I am having a problem with Private Messaging where it is not displaying properly. I have tested on my dev environment and the issue is still present, with all plugins deactivated except WPMU. I will attach a screenshot of the problem I'm having.

Also. Registrations on our website are closed. The only way to gain access to our website is either by signing in with a social media login or by making a purchase.

The private messaging screen that I have set up, shows a logged-out user an option to resister for site access. Can we remove this function? I do not wish for logged-out customers to be presented with an option to register on the website. I would only like a login screen to be displayed on this page.

Thanks!