Could you provide more details of what you are doing and what happens?

Also, could you share a screenshot of what you see here?

I'm in Special:UserLogin, I put my password and press Log In, but when I press Log In it doesn't respond and shows the message in MediaWiki:Sessionfailure. Here's the screenshot of the SameSite thing.

What browser and OS are you using? Any special settings (third-party cookie blocking, some privacy extension)?

My OS is Windows 7 my browser is Google Chrome. I don't have any cookie blocking or privacy extension.

You are using the desktop site, right? Can you do another login attempt and provide the exact time when you did it? Can you retry in incognito mode (with "Block third-party cookies" disabled)? If you have an alternative account, can you retry with that?

Yes, I'm with the desktop site. I tried with my alternative account SRuizR777 in incognito mode at 1:50 UTC. Surprisingly it worked. I tried with my main account SRuizR in incognito mode at 1:52 UTC and it also worked.

At 1:55 UTC I tried out of incognito and it didn't work.

Nevermind, I restarted my computer and now it's working. Kind regards.

Thanks! Please reopen if you see this again; we made some changes to the login system, in response of a change in browser behavior (which is rolled out this week and next) so we are very interested in reports of errors (although I'd mostly expect them to happen around cross-wiki login, not direct login).

name and version of your browser, whether you have used the desktop or mobile site

Desktop - Google chrome 83.0.4103.116

checked the "keep me logged in" option

No

a screenshot of https://samesite-sandbox.glitch.me/ and the name and the first 4-5 characters of the value for all the cookies with "session" or "token" in their name

Maybe it'll not help you now? as I faced it on 2:11 p.m. today. I can't log in through arwiki and enwiki, then I tried through metawiki, so I can log in through metawiki only, and when enter arwiki/enwiki the unified login not work, and I'd log in again through each project alone. So I asked through IRC, and directed me to T258148. I read on it I cleared the cookied for wikipedia.org and was able to login again, so I cleared cookies then restarted the browser. Then all back work fine!

@Aklapper I finally managed to log in, in practise it was my not-updated browser; the current version is 12.1.1.36. Thank you for your time.

I found steps to reproduce this on Firefox 106.0.5 (Release):

1. At about:config, set javascript.enabled to false
2. Log in at https://en.wikipedia.org/wiki/Special:UserLogin
3. Load https://commons.wikimedia.org/wiki/Special:UserLogin

Autologin never happens, and attempts to log in through the form cause sessionfailure. The Special:CentralAutoLogin GETs (checkLoggedIn, createSession, validateSession and setCookies) all occur as they do with JavaScript on.