One user can sync his iPhone and not the other. How do you go about troubleshooting that? You can't find any obvious difference between the two accounts. I was in that exact position last night with some two different accounts.
The symptoms was strange. One account worked just fine and no issues connecting an sync his phone. The other could verify his account information but when he tried to sync a folder, contact or calendar, It just stopped with a simple iPhone message "unable to connect to server". iPhones are easy to use until there is a problem. You cant get much out of an error message relayed to you from a user.
The issue was how ever resolved when I discovered that it was the folder sync that returned 500 internal server error on the second user. I went in to the AD and enabled "Advanced Features" in the "View" menu. Then selected properties on the user, enter the "Security" tab, select "Advanced" and make sure that the "Include inheritable permissions from this object's parent" check box is checked.
Great way to do troubleshooting on this type of issues is to use: https://www.testexchangeconnectivity.com/