Error initializing local changelists : TFS configuration error : Unable to find valid certification

I am a beginner. I have configure TFS in Visual Studio (Collaborator Extension) and everything seems find.

Then i open the Collaborator Client GUI, and the TFS appear under the SCM system list. But when i click the 'Add Shelvesets', an error prompted

"Error initializing local changelists

Reason:

TFS configuration error. sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valis certification path to request for target"

Please help. I really have no idea.