support code 0x86000108 on some accounts

I've set up an HTC Touch Pro for my boss, but cannot sync over the Internet
to Exchange 2003 SP2 because of this support code 0x86000108. However it
will sync the contacts, just not the Calendar or Inbox.

When I provided my credentials it synced with my account without a problem,
and I've set up a new account for him which is working.

I'm not sure how to solve the problem and get his original account working.
I moved the 6,600 items from his Inbox into a subfolder but that didn't solve
the problem. The overall mailbox size is very large (almost 2Gb). Is it
likely that the sync issue is down to mailbox size, or is there some corrupt
item either in the Inbox or the Calendar? Isolating it would be a
time-consuming process...