Please provide us your login page so we can take a closer look, both
/wp-login.php is not working.
Your version of X (5.2.5) and Cornerstone (1.3.3) is not compatible. Please update both to the latest version (X 6.0.0 / CS 3.0.0)
Updating Your Themes and Plugins
Remember to clear all caches (plugin, server-side, CDN, and browser's cache) after updating so that the code from the latest release is always in use. This will help you to avoid any potential errors.