When added to a managed package, only Apex controller methods marked global should be accessible by Lightning components outside of that managed package’s namespace. Methods marked public in their Apex definitions should be accessible only to Lightning components included in the package’s same namespace. When this update is enabled, these rules are enforced.
Prior to this update, non-global methods (that is, Apex controller methods that weren’t marked with the global access modifier) could be accessed by Lightning components outside of the package namespace.
When this update is enabled, if you’ve written code that incorrectly depends on non-global Apex methods in a managed package, server actions that call those methods fail. Update your code to use only the supported API methods provided by the package as global methods.