Why is my main navigation menu disappearing on login?

I'm not sure if this is necessarily a Membership Pro issue, but when my members log in, the navigation menu disappears (across the top). Upon successful log in, it takes them to:

http://aft1481.org/members/blog-1481/

Navigating away from this page brings the menu back, but once they return to the page above, the menu disappears.

Thanks.