Exchange 2003 to 2013 ActiveSync not working for existing users

Having a problem with ActiveSync for existing users in a newly upgraded environment.

Domain has been migrated from 2003 to 2012 with the existing Exchange 2003 installation being removed and a clean install of Exchange 2013.  All is working except for Active Sync.

Using test-activesyncconnectivity cmdlet gives an HTTP 403 Forbidden error with the details showing

_Error:UserDisabledForSync

A test exchange user (created using the new-TestCasConnectivityUser.ps1) passes all the test-activesyncconnectivity tests successfully.

Properties of the existing mailbox show that Active Sync is enabled.  I have tried disabling and re-enabling it but still get the same error.

I've searched but can't find any relevant info on this issue, so any help would be greatly appreciated

Thanks

Jon Reeves