I just got this message after logging in:
Your submission could not be processed because a security token was invalid.
If this occurred unexpectedly, please inform the administrator and describe the action you performed before you received this error.
Are you subsequently having any trouble using the site? What browser and version are you using?
We've only had a couple reports of this happening, and I read on a software help site that clearing your cache should address the issue for the individuals experiencing it.
If that does not help, and you're still having trouble, please let us know!