I have a site with R650 APs still on a ZoneDirector 1200 running 10.5.1.0 Build 199 that all eventually show up as 'disconnected' in the ZD after an undetermined period of time.
The APs and ZD are in the same vlan and IP subnet and the only changes recently were the addition of the R650 APs (previously R500). In this state the APs still broadcast SSIDs but won't get any config changes from the ZD, they are still on the network, reachable, and they can ping the ZD and the ZD can ping them. From the AP-CLI 'get director' shows the correct IP & MAC Address.
So far my solution is SSHing into the APs and rebooting. Anything I can check or try to resolve this long-term?