First of all, check the news section of the game's website: https://warthunder.com/en/news and make sure that no maintenance work is being carried out in the game.
If no maintenance is in progress, then to isolate the issue, try logging in to the website https://store.gaijin.net/.
If login is unsuccessful, try changing your account password using the password recovery system: https://login.gaijin.net/ru/sso/forgotPassword.
Next, consider some possible solutions if you can log in to the website but the error still occurs when logging into the game:
- If you are using any antivirus software, add the game folder to its exceptions.
- Run a diagnostic of your connection as described in this article. Pay special attention to items 4, 5, 6, and 7, and carry them out.
- Try changing the DNS servers you use.
If none of the suggested options help, contact support, attaching all the files you obtained while following the steps from the article in point 2.