Yeah, something seems to be going horribly wrong with the session caching, though I'm not yet sure what. I don't think we've ever had a MediaWiki upgrade that hasn't gone awry, perhaps we should consider switching to another CMS.
I've just tried fiddling with the caching options, and the error hasn't reoccurred over 10 minutes of testing, though that proves nothing since the issue was intermittent beforehand. If it's still happening, please let me know.
We'll keep investigating and looking for a proper fix.