It happened to me today while trying to login on meta or enwiki: Exception encountered, of type "Exception"
I've seen this stack in the log:
2016-05-26 22:58:28 [V0d-lApAMFwAAFzfE8cAAAAJ] mw1257 enwiki 1.28.0-wmf.3 exception ERROR: [V0d-lApAMFwAAFzfE8cAAAAJ] /w/index.php?title=Special:UserLogin&action=submitlogin&type=login&returnto=Main+Page Exception from line 2313 of /srv/mediawiki/php-1.28.0-wmf.3/extensions/CentralAuth/includes/CentralAuthUser.php: Could not find local user data for DCausse (WMF)@tawiki {"exception_id":"V0d-lApAMFwAAFzfE8cAAAAJ"}
[Exception Exception] (/srv/mediawiki/php-1.28.0-wmf.3/extensions/CentralAuth/includes/CentralAuthUser.php:2313) Could not find local user data for DCausse (WMF)@tawiki
#0 /srv/mediawiki/php-1.28.0-wmf.3/extensions/CentralAuth/includes/CentralAuthUser.php(2253): CentralAuthUser->localUserData(string)
#1 /srv/mediawiki/php-1.28.0-wmf.3/extensions/CentralAuth/includes/CentralAuthUser.php(2213): CentralAuthUser->queryAttached()
#2 /srv/mediawiki/wmf-config/CommonSettings.php(399): CentralAuthUser->getLocalGroups()
#3 /srv/mediawiki/php-1.28.0-wmf.3/includes/Hooks.php(195): Closure$wmfLoadInitialiseSettings#2(User, array, string)
#4 /srv/mediawiki/php-1.28.0-wmf.3/includes/password/UserPasswordPolicy.php(150): Hooks::run(string, array)
#5 /srv/mediawiki/php-1.28.0-wmf.3/includes/password/UserPasswordPolicy.php(75): UserPasswordPolicy->getPoliciesForUser(User, string)
#6 /srv/mediawiki/php-1.28.0-wmf.3/includes/user/User.php(1071): UserPasswordPolicy->checkUserPassword(User, string, string)
#7 /srv/mediawiki/php-1.28.0-wmf.3/includes/user/User.php(4272): User->checkPasswordValidity(string)
#8 /srv/mediawiki/php-1.28.0-wmf.3/includes/specials/pre-authmanager/SpecialUserlogin.php(817): User->checkPassword(string)
#9 /srv/mediawiki/php-1.28.0-wmf.3/includes/specials/pre-authmanager/SpecialUserlogin.php(1064): LoginFormPreAuthManager->authenticateUserData()
#10 /srv/mediawiki/php-1.28.0-wmf.3/includes/specials/pre-authmanager/SpecialUserlogin.php(367): LoginFormPreAuthManager->processLogin()
#11 /srv/mediawiki/php-1.28.0-wmf.3/includes/specialpage/LoginSignupSpecialPage.php(1410): LoginFormPreAuthManager->execute(NULL)
#12 /srv/mediawiki/php-1.28.0-wmf.3/includes/specialpage/SpecialPage.php(479): LoginForm->execute(NULL)
#13 /srv/mediawiki/php-1.28.0-wmf.3/includes/specialpage/SpecialPageFactory.php(591): SpecialPage->run(NULL)
#14 /srv/mediawiki/php-1.28.0-wmf.3/includes/MediaWiki.php(282): SpecialPageFactory::executePath(Title, RequestContext)
#15 /srv/mediawiki/php-1.28.0-wmf.3/includes/MediaWiki.php(746): MediaWiki->performRequest()
#16 /srv/mediawiki/php-1.28.0-wmf.3/includes/MediaWiki.php(520): MediaWiki->main()
#17 /srv/mediawiki/php-1.28.0-wmf.3/index.php(43): MediaWiki->run()
#18 /srv/mediawiki/w/index.php(3): include(string)
#19 {main}Explicitely login to tawiki fixed the issue.