Description:

This error is a result of some security certificates expiring.

First solution:

 - Launch the Firestorm launcher,

 - Choose your extension and locate the game on your computer,

 - Select "Options" and click on "Check and repair",

 - Wait for the repair to be completed,

 - Try to connect by clicking on the "Play" button.

Second solution:

 - Close the game completely,

 - Delete your folders: Cache and WTF,

 - Test your game.