When trying to set DCA interval from the WLC GUI this error can be seen. On 8.2 this can be happen when you try to set a interval higher that the FRA (flexible radio assignment) interval. Even if FRA is disabled!
Trying to change to setting fra CLI will show show the reason
(Cisco Controller) >config advanced 802.11b channel dca interval 12 DCA Interval cannot be greater than FRA Interval.
If FRA is disabled it is not possible to set the interval from GUI. To fix the problem login in via CLI and to the following
(Cisco Controller) >config advanced fra interval 12