This week I received an email telling me that they got an error while using the MS Tag REST  service:

System.ServiceModel.Security.SecurityNegotiationException: Could not establish trust relationship for the SSL/TLS secure channel with authority ‘ws.tag.microsoft.com‘.

As you can see the authority is complaining about the Microsoft domain. The issue was the certificate used by Microsoft’s Tag API was outdated and thus the error was thrown when the  REST service communicated with the Microsoft service.

Since the certificate has been renewed and is now valid till somewhere early 2011 this problem should be resolved.