Android on Exchange - calendar keeps having a sync error

We run a small business server 2003 with Exchange. We have one user that seems to consitently have a problem with syncing his Calendar.

This problem happened with the Palm Pre once before but has now happened twice in the week with his new HTC Evo (Android). There are 6 other users in the office with the same phone and no problems.

Once the account is setup on his phone, everything works fine. Then, a few days go by and all of a sudden there will be  sync error on only the calendar part. To try and resolve this, we have gone into Outlook and removed everything from his calendar and tried to sync with no success. We have removed his account from the phone and re-added which does not fix the problem.

The only solution so far (which we should not have to do) is to backup all his info using Outlook and then delete his mailbox in Exchange and then create a new mailbox and copy all the data back over. At that point, everything works fine for awhile...

SO, I'm trying to find out if anyone has any ideas on what might be causing this and/or how to permenantely fix this. When we do a manual sync, the EVO immediately shows a sync error for the calendar - doesn't even seem that it has enough time to try and sync - which makes me wonder if its an Exchange issue (ie corrupt mailbox) or an Android issue.

Any suggestions for a solutions and/or ways to figure out just what is causing the sync error would be appreciated.

Thanks in advance!