iphone randomly locks active directory account

I found an old thread:https://discussions.apple.com/thread/1713156 and the last few comments seem to be exactly the problem my users are currently experiencing, however, no answer to their question was ever posted. I don't know if they received a response from Apple directly or how they resolved their issue.

My issue is that a handful of users with iPhones will randomly get locked out of their active directory account. They can be surfing the net or browsing email when suddenly they receive a message that "the username and password for exchange is incorrect". As soon as I unlock his account the server starts pushing mail to him. The fact that he receives mail without typing anything after I unlock his account indicates that the information in his phone is correct.

So far I have had him remove and re-add his exchange account, turn off his wi-fi, turn off iCloud. I am thinking about doing a factory reset, but his phone is brand new. We received it, set it up, updated it, added his account. Does anyone have a suggestion?

Thanks in advance!