Forwarding a subdomain produces a blank page

I have a forwarded, masked subdomain of mysite.domain.com that is pointing to the home page home.multisite.com of one of my multisite sites that is set to private requiring a password. If I navigate directly to the actual address of the site, home.multisite.com, the site works beautifully asking for a login, etc. However, if I navigate using the forwarded subdomain that is pointed to that same page that just worked beautifully, I get just a blank white page. I even tried changing the forward to point straight to login page, not just the home page still producing a blank white page. I don’t have a domain mapping set up for this since it is just a forwarded subdomain, but wasn’t sure if that could be the issue. Does anyone know why I would be getting a blank page and, if so, how to fix it?

Thanks!