HTTPWebRequest target principal name error

Benno67

Hi,
We are trying to get a webservice call from NAV to NAV in NAV2013R2 (to synchronize contacts between company's).
We did this before using automations, but for NAV2013 we need to use .Net.
So we took Gunnar's sample, modified it for utilizing XMLports, and get the following error :

System.Net.HttpWebRequest.GetResponse failed : The target principal name is incorrect

Checked the starting of the service, no warnings, service principle names are correct assigned.
Url is working from IE, we can see the services.

The only thing we can find on 'target principle names' has to do with certificates and ssl??

Also tried Gunnar's sample 'as it is', but the same result

Something to do with NAV2013R2 Hotfix 6 we are using?

Any suggestions are welcome.

Benno