iPhone 5 problem Exchange sync over WiFi

Hi, I am having a problem syncing my new iphone 5 with Exchange 2010 over WiFi. It works fine on 3G. I am on the latest IOS release 6.1.3. Anyone seeing similar problems? Other stuff works fine over WiFi, including Gmail and Yahoo mail, just ActiveSync fails. I tried deleting the Exchange account and recreating, and that failed over wifi too! had to switch to 3G to create the account! Same symptoms with different wifi networks (work/home/public).

Thanks for any help,

Moshe