You had your caching set to display cached pages to logged in users … so, they were seeing the logged out cached page instead of the real page when logged in. Generally, you never want to serve cached pages to logged in users, for this reason. Since you have an “online store” of sorts this could be doubly bad. If a page is cached when a user is logged in, that cached page can be served to ALL visitors potentially exposing that users details and such to everyone until the cached page expires.
Hope this helps!