Hi,
It manifests differently, sometimes one cannot login, unless he checks "Remember me" but that is fake login, not allowing to post. If one is already logged, posting is impossible and results into "Submitted form is invalid" error. But accessing a user control panel still works but marking forums as read does not.
It seems to be always somehow related to cookies (and the SID being in the URL when it does not work) and fixes itself in a few hours.
Regardless, I find it quite annoying, as it happens often.
Regards,
PB