This worked for me:
1). Push the Windows Key + R and paste the following with or without the quotes will work "%appdata%\wargaming.net\WorldOfTanks\" (NOTE: This is the World of Tanks cache folder.)
2). Delete everything in this folder except the preferences.xml (this file contains all your game settings/hud/cursor customizations etc.
3). Start the game.
4). Missions menu should now work just fine!