Wireless LAN Controller AireOS software releases prior to December 2013 are in the processes of being deferred due to the following bug:
CSCul68057 CF driver change for 5508/WiSM2/2504
This bug affects 2500, 5500 and WiSM controllers, and is fixed in the following releases of AireOS:
For a complete fix, upgrade to FUS (Field Upgrade Software) 184.108.40.206 as well.
For further guidance on AireOS software selection, see the Product Bulletin Guidelines for Cisco Wireless Software Release Migration.