I don't think there is a solution. LDAP uses basic authentication, which means username and password go over the wire with a base64 encoding, but no encryption. Microsoft Office does not like this and prevents the request going through. On MWG there is nothing you could change apart from changing the authentication mechanism, so the only way would be to apply the change to the client application.
- You could switch authentication from direct proxy authentication to authentication server. This will allow to securely transfer the LDAP credentials via HTTPS. Note that this is a change that must be planned and tested
- You can check the access.log and see if the office requests come with a different user-agent. If there is a user-agent that is special for those requests, skip authentication for them