Notification code: PROXY_UNLICENSED?

Question:

Symptoms: When I try to access a website through the Cisco Web Security Appliance (WSA), the proxy displays an error message that says Proxy Unlicensed:

This Page Cannot Be Displayed
----------------------
Internet access is not available without proper licensing of the
 security device.
Please contact your corporate network administrator and provide
 the code shown below.
Note: To access the management interface of the security device,
 enter the configured IP address with port 8080.
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Notification code: PROXY_UNLICENSED

The feature keys are not expired. What's the problem?

Environment: Cisco Web Security Appliance (WSA), all versions of AsyncOS

The EULA (end user license agreement) was not accepted, but should be.

You can achieve this acceptance two ways:

1. Running the System Setup Wizard and going through the guided setup process: 'System Administration' tab > 'System Setup' > 'System Setup Wizard'.
2. Go to 'Security Services' tab > 'Web Proxy' > 'Edit Settings'. If the EULA appears, it has not yet been accepted, this still needs to be done. If clicking on the Edit Settings button allows you to edit the proxy options, it has already been accepted.