Read this if you can't log in

Eternal-

According to the idea, solutions to your problems should be collected here, lets see how it will go.

Firstly, check these endpoints and compare the expected results:

https://hydra.faforever.com/oauth2/token -> Method Not Allowed
https://hydra.faforever.com/oauth2/auth -> The OAuth2 request resulted in an error.
https://user.faforever.com/oauth2/login -> {"errors":[{"errorText":"400 BAD_REQUEST...
https://user.faforever.com/oauth2/consent -> {"errors":[{"errorText":"400 BAD_REQUEST...

if some page is unavailable, or returned a different result, you should check your connection for restrictions because you cant access this endpoints = client dont have to them either

Turn off antiviruses, firewall, check router/wifi restrictions...

Secondly, check localhost for availability

Win+R
Write "cmd" and press enter
Write "ping localhost" and press enter

Result: lost packets should equal 0

if there is packet loss, then your localhost have been blocked.
Reasons:

You setup it like this
Computer politic
Firewall settings

by Marc-Spector

Solutions: from forum

Turn off VPN for browser
Turn off or select another country for desktop VPN/proxy
Install and test old client GitHub sources: https://github.com/FAForever/client
Install and test version not older than 2021.10 of official client GitHub sources: https://github.com/FAForever/downlords-faf-client

Incoming...