The client you are using to log in to the game is out of date or there is a mismatch between the client and server versions. This error can occur if there was a recent update to the game and you haven't patched. Follow these steps to resolve the issue:
- Use the Battle.net Desktop App to launch the game. Any pending game updates will download once the application is opened.
- Try repairing the affected game to make sure there are no corrupted game files.
- If the issue persists, uninstall the game client and download a new copy of the installer from the Battle.net Game Downloads page.