This is a bit strange. I can translate and modified existing translations, but I can't mark translations as reviewed because it says that my session is closed. But it isn't. Proof ot this is that my translations in the same browser, in the same session, at the same time are saved with my username. I tried closing and opening again my session, but it didn't work. I tried Chromium, Chrome and Iceweasel (Firefox in Debian).
I can't say exactly when this started happennig. Maybe a week or two. Before that, I had no trouble.
Let me know if more information is needed.
Update: I didn't specified the site: it's right here in www.mediawiki.org. I have no trouble in meta (see today log)
Update 2: This also happend to me in translatewiki.net. Could it be the translate extension version?
Update 3: It's already been MediaWiki:Tux-session-expired_shown_when_Reviewing_a_message_44515 reported in translatewiki.net by other users.