I have both Eclipse version 4.6.3 & 4.13.0 installed. As well as STS 4.8.1.RELEASE. The 2 Eclipse versions can connect to Eclipse Marketplace with no issues. But I get the below error when connecting from STS. I have compared network setting including my proxy. It all looks the same.
MarketplaceDiscoveryStrategy failed with an error Cannot complete request to https://marketplace.eclipse.org/api/p?client=org.eclipse.epp.mpc.core&os=win32&platform.version=4.17: Unable to read repository at https://marketplace.eclipse.org/api/p?client=org.eclipse.epp.mpc.core&os=win32&platform.version=4.17. Unable to read repository at https://marketplace.eclipse.org/api/p?client=org.eclipse.epp.mpc.core&os=win32&platform.version=4.17. PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target