Invalid Form error

This issue only just started up and only effected one of the websites I run: http://fear.monochromestar.com/

When I tried to upload a new page, and when I tested to add a marker and new menu link, it took me to a blank page that just said "Invalid form" and didn't save anything.

I was able to resolve the issue by logging in again, but it seems like something that should be mentioned since I don't know what caused it.