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It's quite possible that when you enabled cached exchange mode, you also made the Permissions for the folder and/or files involved in computer and phone synchronizing, too high for your customer. Check the permissions of those files, and those files/folders Owner, to make sure that isn't the case.
In cached exchange mode, you may find that user names are ***** ***** in the GAL. If that's happening, it could prevent your client's users from syncing.