Mac 10.11 (El Capitan) - Repeating system.log error: airportd[57]: SecTaskLoadEntitlements failed error=22

To answer your question, here's an introduction to what SecTaskLoadEntitlements really is/does:

An application (.app) in OS X is really a folder containing a number of files, including the actual program, resources (graphics, sounds, etc) as well as system files such as plist files.

One of these plist files contains a descriptions of the application's entitlements. An entitlement is either that the application "can do" a certain thing, or that it has permission to do a certain thing.

Examples on OS X would be for an app to have the permission to get read-only access to your Music folder, or that it is allowed to interact with Bluetooth devices, record from your microphone, etc.

Application on the OS X App Store are required to use entitlements in so that that they're required to be sandboxed, which is an entitlement in itself.

In order to make sure that entitlements cannot be changed or unrightfully gained, the entitlements are "signed". This means that the developer of the application uses a specific cryptographic key to "sign" the entitlements.

OS X checks the signature when loading in new applications, and only if it is signed correctly will it load the entitlements and apply them to the program.

Your error message means that the "airportd" program tried to load in the entitlements for an application, but it failed with error number 22. The number 22 means that an invalid argument (parameter) was given.

Airportd manages the wireless interfaces (WiFi) on your Mac.

So to sum up: The source process is "airportd", and the issue is an invalid argument given for loading entitlements.

However, this does not mean that you'll experience any user-facing errors. If you're not having any problems with the WiFi interfaces, then you can just ignore the log messages.