Certificate check error.
Please, make sure that file /etc/hosts wasn't changed, and also check you computer for viruses.
Sometimes this issue appears if an antivirus, a firewall or other software applies some changes to the SSL certificate. In this case turn off the SSL check function of your antivirus. Make sure that War Thunder isn't blocked by a firewall, add the game to the list of exceptions of your firewall or simply disable the software.
You might need to disable the built-in Windows firewall by following the steps:
Open Windows Firewall by clicking the Start button , and then clicking Control Panel. In the search box, type firewall, and then click Windows Firewall.
Click Turn Windows Firewall on or off. If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
Click Turn off Windows Firewall under each network location that you want to stop trying to protect, and then click OK.
If you manage to find out that the reason of your issue is not your software, please contact your ISP - some of them substitute SSL certificates.
If nothing above helps, please create a Support ticket and attach your game logs(.clog files) placed at \War Thunder\.game_logs
To make your ticket more descriptive, attach your DxDiag file to it.