HI,
I have a problem with that too. LDAP authentication is working fine with mobile devices (Android, iOS), but is not working with laptops (Windows 7, Windows 8). I checked windows 7 and windows 8 configuration, and they work fine when i use LDAP authentication using Aruba Controller (against the same LDAP server). Problem is just with Aruba Instant AP.
If you want to use LDAP for 802.1X for authentication, make sure you enable termination on the WLAN setup on the IAP.
You can't select LDAP server if you don't enable termination
Regards