MultiSite Privacy Plugin doesn't allow access to subdomain

Hello,

I'm trying to put password protection on private.antoneranch.com - a subdomain of the main site. The MultiSite Privacy Plugin seems great, but when I activate it for private.antoneranch.com and then try to login - the login page refreshes endlessly. In other words, I put in the correct password but then am always returned to the login page without actually accessing the content on the subdomain.

I'm using WP Engine for hosting, so I'm wondering if their cacheing could have something to do with it.

Also, if I can get the Multisite Privacy to work, will this subdomain still "discourage search engines from indexing" it? That would be great.

thanks for your help,
Jacob