I had this problem with 3.1.2 as well. It occurs for me only when this plug-in is activated and I am editing a post. I'll hit the preview button and reach the page preview without any problem. When I attempt to go back to edit the post, I'm kicked back to the Administration log-in screen and have to log in again. De-activating the plug-in takes care of the problem. So to edit posts I have to first de-activate your plug in and the activate it again when I'm done.