Summary:

When synching contacts, an error was being logged by the Exchange sync engine stating that an element was failing validation.

Issue:

The element was as follows:

http://schemas.microsoft.com/exchange/services/2006/types:BusinessHomePage

A value of 'http://' was not valid.

Resolution:

The customer had set a pers_website value of "http://" for a large number of their Person records (possibly by import).
A number of these Person records had been added as user contacts. Since "http://" is not a valid URL, EWS was rejecting the value when the synch was run.

NULLing the invalid pers_website values resolved the issue.