No, this has not happened before.
If I'm logged in but been inactive for 20 minutes and want to post a story or edit a static page, I have to re-authenticate to get to the story/static page editor itself. Yes?
If I submit the story two minutes later, I get the security token invalid message. That has not happened since pl5.
I have 3600+ stories in the system, this behavior started a few days ago.