When you have an SSL enabled website and enforced. When logged in as a host user if you want to use the file manager the page is nonSSL and gives an antiforgery browser exception.
The cookie is not being passed between SSL and nonSSL site pages to allow access.
This page should load with SSL but I think because it does not come SSL enabled and the site is SSL enfroced we get this error.
I believe maybe moving the file manager to the persona bar would make sense? It would be nice not to have to load another page to add or manage files.