I upgraded my C25 to version 9.15.02.0009 and am having a new issue with HP Chromebooks on AP3825i AP's. A particular device will appear associated with an AP in the Mobile Clients section of the website with a 5.0n connection but are not able to get to the network. If I Disassociate the device from the controller's web interface, the Chromebook immediately connects at 2.4n and I can get onto the network.
Can you confirm what the 5ghz radio is configured for? That is the 2.4 side. If you would like to troubleshoot this out of the forum please feel free to contact me directly dhyde@extremenetworks.com.