Since yesterday (not sure if this is a coincidence, but from when someone requested a password reset of an account created with my primary account), I have been logged out of at least Commons, Wikidata, Wikipedia and Wikisource getting the following error message when trying to log back in:
There seems to be a problem with your login session; this action has been canceled as a precaution against session hijacking. Go back to the previous page, reload that page and then try again.
At the same time, I have not been logged out of at least Meta, MediaWiki.org, Wikivoyage and the German Wikipedia, where I can still log out and back in freely. Emptying the cache didn't make a difference, while using my browser's private mode did (perhaps a corrupted cookie?).