For those users who are experiencing the "Error 419 - Page Expired".
This is a known issue and we're working on a solution.
It typically happens on the post/reply pages, maybe on some other pages like Login/Register. As far as we can tell, it happens when the page has been idle for too long, and can be prevented/corrected by refreshing the page. We know that's an inconvenient band-aid and are working on a permanent solution.