Some customers have experienced problems with the SA520’s RADIUS server. There is a firmware update that resolves the issues. You can download the latest software here: http://www.cisco.com/cisco/software/navigator.html?mdfid=282414017
You can get more information about the update in the Release Notes, which you can find here: http://www.cisco.com/en/US/docs/security/multi_function_security/multi_function_security_appliance/sa_500/release/sa500_MR3_v2.1.18.pdf.
Moving document to Small business Security where it is a better fit.