It can be caused by the spam verification system. I've never had it happen on my system, but best I can tell there is an issue with the current login and the board cookies.
I am assuming you are using a single computer.
I suggest logging out of the forum and the clicking the "delete board cookies" like on the main page.
Shut down your browser (not really required strictly speaking, but to ensure complete cookie removal).
Restart browser and then login. Hopefully that will have fixed you.