I know this is an old thread but it was my first hit when researching the issue. My issue occured on one VAP after we rebooted the master controller for maintnance reasons (The controllers SNMP was still telling our monitoring system that APs existed on the controller when they had already been deleted. We are running 6.4.4.8 on Aruba7210-US)
Found my solution in 6.4.4.6_Release_Notes.pdf
Symptom: A client fails to connect to an SSID. The log file for the event lists the reason as Capability requested by STA unsupported by AP. Scenario: This issue occurs during a failover in a High Availability (HA) setup, when no Virtual Local Area Network (VLAN) is assigned for a Virtual Access Point (VAP) profile and the VAP is configured in tunnel mode. Workaround: Configure a VLAN ID in the VAP profile by using the vlan CLI command.
I looked at the VAP and the VLAN had disappeared. once i readded the vlan all the clients instantly connected.