At the building where we are experiencing the issue, we have eighteen Aruba IAP-105US access points. We are getting a high number of "DHCP Request timed out" alert messages, it seems to happen especially in the morning when a lot of our users are connecting for the first time. DHCP is provided by a Windows Server 2008R2 domain controller.
I have confirmed that the switch ports the access points are on are configured for the appropriate VLANs (knowing that DHCP requests would be denied if these were configured incorrectly) and confirmed that there are enough DHCP addresses to accommodate the clients. This issue seemed to occur shortly after I upgraded Instant to 4.0.0.5 on this network (other networks of ours are fine on that firmware); as a result, I rolled back to 4.0.0.4. This did not resolve the issue, apart from a brief period of perhaps 1-2 days after the wireless network was rebooted due to the rollback.
If anyone has any suggestions, I would appreciate it.