After logging into the forum, the home page does not reflect the successful login (still shows the login button, etc). The http headers also seem odd on the home page (last-modified date isn't change).
If I click into a forum or thread, it works as expected. Something is going on with the index page.
I have restarted memcached as suggested in a previous post. That did not affect anything.
There appears to be a layer of caching going on above XenForo (or from an add-on). I just had your main forum page return a 304 not modified and that's not something XF would ever do itself on this page. That would be the cause of this issue.