If you are encountering this error, please check whether the server is undergoing maintenance. News about maintenance work is posted on the website https://warthunder.com/en/ and on the forum https://forum.warthunder.com/ .
If you have any antivirus software installed, add the game folder to its exceptions.
We also recommend checking the most common issue. Please read this article and make sure that all the ports mentioned in it are open.
If the problem persists, please submit a ticket to our support service. Our specialists will help resolve it.