For the past four days, site logins have been failing during the part of the process which logs you into the wiki. We believe we've solved the problem now.
The cause was an incompatibility between the LdapAuthentication extension for MediaWiki and PHP 5.6.11+. We upgraded to PHP 5.6.12 from 5.6.10 on Wednesday night, but didn't detect the problem ourselves because our login cookies were still valid from before the upgrade. Thanks for your patience, and to everyone who reported the problem.