I encountered this problem today, “Session verification failed. Please try logging out and back in, then try again.”, though this problem can be solved simply just by logging out and back in (logged out/in fixes it), but just wondering why it happened this time?
This is not a bug, it is something that happens on practically all user interaction sites
When we enter the page, it is sent from the server to the user's browser.
Then the server waits for the response, in this case from a post.
But this has to be done within a certain period of time, otherwise the connection between the browser and the server disconnects.
This happens for security reasons, in order to prevent abuse or data theft.
So, when you have the window open for a long time and then click on make post, this message appears, so that that page reconnects to the server.
(Note that this is a simple explanation of how it works, without getting too technical.)