The way you fix this problem (I've had it twice (once for each pack I've bought)) if you are launching through steam is to do the following:
C:\Users\User\Documents\My Games\WarThunder\Saves and edit the profile.config file with your favorite text document editor thingy (I use notepad).
Change "USEROPT_AUTOLOGIN:b=yes" to "USEROPT_AUTOLOGIN:b=no" so that it doesn't try to auto login anymore.
That's it. Now you're good to go. Once you log in once, you're free to set it to auto login again. No idea why it happens, but at least it's fixable.