Error 12- This error is caused by the Global Play feature being disabled until all regions are active. Please make sure you choose the correct region when starting the game as per this sticky:
Error 37 - The server is full. This is likely due to high login traffic. The only solution is to keep trying to log in.
Error 315300 - If you're sure you're typing your correct account information, check to make sure no additional spaces were added before or after your email address. If you're still running into this issue, there may be an error in your foreign language appdata files. Some players have found a workaround, but please be aware the steps they provide are not something we can currently support.