ERROR: unable to find valid certification path to requested target

Hi I have been downloading MODIS data using the Modis Client with Matlab.

It has been working perfectly, however now when calling the modis Client: modisClient() the following error appears:

 Error using callSoapService (line 89)
javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed:
sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

 Error in myServiceHandlerService/getproducts (line 22)
response = callSoapService( ...

 Error in modisClient (line 98)
  modisProds = getproducts( modisServiceObj ) ;

Any suggestions on how to fix this?