Cisco's 12.3.(8)JA firmware included changes to its SNMP MIB. Because of these changes, AMP versions 4.1.2 and older couldn't fetch user and bandwidth data from APs with this firmware and therefore considered them "Down".
If you get this error on IOS APs with 12.3.(8)JA or newer, upgrading to AMP 4.2.3 or newer will resolve the problem.