I don't know, it seems like a bug so if you send in a ticket with a detailed case on how to reproduce this someone might be able to have a look at what's going on. You can also set debug mode in the advanced tab and another tab will appear in your my account form with object ids. Maybe the (difference in?) ids will show you something.
That said, if you can get your customer to upgrade to Exchange 2007 or up, that's an even better option. Support for web access and synchronization is not nearly as good in Exchange 2000/2003 as in later versions which use web services.