The NM-AIR-WLC6-K9 WLCM module is not supported beyond the 18.104.22.168 release. It is not supported or tested in 5.0 or above.
We now display the following banner in Cisco Wireless LAN Controller Module Download Software section on Cisco.com.
Warning: the Wireless LAN Controller Network Module (NM-AIR-WLC6-K9) is not supported in any software release after 22.214.171.124. Attempting to install 5.0 or later software can permanently damage the module.
If you are running 5.x, 6.x or even 126.96.36.199 build of software on your NM-AIR-WLC6-K9 and planning to install the latest software and find that it does not successfully install or that it installs but will not let APs join or will be rendered inoperable. In such cases, Please contact TAC or downgrade to 4.2 software.