This is because the 5.7 license keys have changed and need to be manually updated. To correct this, follow these steps:
1.) Log into your client area
2.) Copy the license key for the correct domain
3.) Open admin/includes/config.php
4.) Paste the license key in the appropriate spot and make sure your changes are saved
You may need to log out of the application and clear your cache for the changes to take effect. Please let me know if you have any questions.
