Unable to read repository at https://tools.hana.ondemand.com/luna/content.xml

Hi, I am following this tutorial:

SAP HANA Cloud Platform

"Installing SAP Development Tools for Eclipse"

configured my proxy settings (I am sure the data is correct, ip, port, user, password).

and go to Help, Install Software and paste the url SAP HANA Cloud Platform

and get this error:

Unable to read repository at https://tools.hana.ondemand.com/luna/content.xml.

Unable to read repository at https://tools.hana.ondemand.com/luna/content.xml.

sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

Any advice how to fix this error?

Thanks.