Over the past 48 hours, there seems to have been a change in the behaviour of MediaWiki login API for Wikidata. This causes all logins from Wikidata-Toolkit (Java library) to fail. Among others, this affects OpenRefine users.
The bug comes from the fact that Wikidata started returning cookies using a set-cookie header, whereas Wikidata-Toolkit expects Set-Cookie. HTTP header keys are case insensitive so Wikidata-Toolkit should recognize set-cookie as well, but as things stand this breaks login in all clients relying on this library. See this pull request on Wikidata-Toolkit for details.
While we can of course fix things on Wikidata-Toolkit's side, it would be great if the change on Wikidata's side could be reverted at least temporarily, since fixing this on our side requires making new releases of Wikidata-Toolkit and OpenRefine.