As a response to a security incident, we will be logging out all users shortly. [1] We don't think the incident had real-world impact but prefer to err on the side of security. After considering various aspects (such as the effect on upcoming software deployments, and finding a time when most people are asleep), we decided to do this on very short notice. I apologize for the disruption.
You can find more information at https://phabricator.wikimedia.org/T124440 If this causes any unexpected problems (apart from having to log in again), please report them there.
[1] We have actually done so progressively over the last week, but that method turned out to be unacceptably slow, and we found a way to do it all at once.
-- Gergő https://www.mediawiki.org/wiki/User:Tgr_(WMF) WMF Reading Infrastructure