From Logstash around the time I logged in (today around 18:00 UTC):

Xl-s2wpAMNIAAftEcsgAAABP (2020-03-04T18:01:03, POST /w/index.php?title=Special:UserLogin):

• [CentralAuth INFO] authentication for 'Krinkle' succeeded
• [CentralAuth INFO] Already fully migrated user 'Krinkle'

Xl-s7gpAICwAAFl9AjsAAAAM (2020-03-04T18:01:18, POST /w/index.php?title=Special:UserLogin)

• [authentication INFO] OATHAuth user Krinkle entered a valid OTP from …
• [authentication INFO] Login for Krinkle succeeded from …
• [goodpass-priv INFO] Login succeeded for elevated Krinkle from …
• [authevents INFO] Login attempt {event: "login"}

Xl-s8QpAICIAAC7qGr0AAAAG (2020-03-04T18:01:21, GET /wiki/Special:CentralLogin/complete?token=…)

• [session WARNING] Session "…": Metadata merge failed: {exception}
• [CentralAuth INFO] Expected key centralauth:central-login-complete-token:… found.

Session "[50]CentralAuthSessionProvider<+:135822:Krinkle>…": Metadata merge failed: MediaWiki\Session\MetadataMergeException: Key "CentralAuthSource" changed

Stack trace:
in /srv/mediawiki/php-1.35.0-wmf.22/includes/session/SessionProvider.php:205
#0 /srv/mediawiki/php-1.35.0-wmf.22/includes/session/SessionManager.php(614): MediaWiki\Session\SessionProvider->mergeMetadata(Array, Array)
#1 /srv/mediawiki/php-1.35.0-wmf.22/includes/session/SessionManager.php(485): MediaWiki\Session\SessionManager->loadSessionInfoFromStore(Object(MediaWiki\Session\SessionInfo), Object(WebRequest))
#2 /srv/mediawiki/php-1.35.0-wmf.22/includes/session/SessionManager.php(191): MediaWiki\Session\SessionManager->getSessionInfoForRequest(Object(WebRequest))
#3 /srv/mediawiki/php-1.35.0-wmf.22/includes/WebRequest.php(813): MediaWiki\Session\SessionManager->getSessionForRequest(Object(WebRequest))
#4 /srv/mediawiki/php-1.35.0-wmf.22/includes/session/SessionManager.php(129): WebRequest->getSession()
#5 /srv/mediawiki/php-1.35.0-wmf.22/includes/Setup.php(798): MediaWiki\Session\SessionManager::getGlobalSession()
#6 /srv/mediawiki/php-1.35.0-wmf.22/includes/WebStart.php(89): require_once('/srv/mediawiki/...')
#7 /srv/mediawiki/php-1.35.0-wmf.22/index.php(44): require('/srv/mediawiki/...')
#8 /srv/mediawiki/w/index.php(3): require('/srv/mediawiki/...')
#9 {main}

A couple things I've tried to poke:

• Grepped objectcache logs for possible timeouts to new sessionstore - nothing.
• Tried to reproduce - logged out and was able to successfully log in, so at least the issue is not persistent.

@Krinkle were you able to login eventually? What steps did you take?

This means that the session logic expected to find a CentralAuth session but found a local one (different cookies in the browser). Visit e.g. https://en.wikipedia.org/wiki/Special:UserLogin in incognito mode, you'll get the usual wiki session cookie, enwikiSession, and none of the centralauth_* cookies you'd see logged-in, nor any user cookies. That's the local session (needed for the login form CSRF check), which is replaced with the cookies for a CentralAuth session in the reponse to posting your credentials; but then that response also redirects you elsewhere to set up the global CentralAuth session, so maybe the cookies don't stick?

In general, see Manual:How to debug/Login problems for what data is typically needed to retrace what happened.

@Krinkle can you update the priority for this task?

Seems similar to T112730: Failure to OAuth after login on mobile.

In T246943#7970467, @kostajh wrote:

Seems similar to T112730: Failure to OAuth after login on mobile.

That one is probably related to the desktop and mobile domain getting mixed up during the CentralAuth login redirect sequence, and some cookie getting lost as a result.
This might be the same issue, but also simply a session timeout during the 2FA step (secondary authentication).