DS-5 v5.28.1: Can't install marketplace plug-ins

I tried installing the ReST Editor plug-in from the Eclipse Marketplace in DS-5, but  I get the following error:

An error occurred while collecting items to be installed
session context was:(profile=SDKProfile, phase=org.eclipse.equinox.internal.p2.engine.phases.Collect, operand=, action=).
Unable to read repository at resteditor.sourceforge.net/.../ReSTEditor_1.0.5.201110101040.jar.
sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
Unable to read repository at resteditor.sourceforge.net/.../org.psem2m.eclipse.rest.editor_1.0.5.201110101040.jar.
sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

This looks like the typical Eclipse "can't find the cert store" issue.

My configuration is:
DS-5 v5.28.1 ultimate in a Ubuntu 16.04 LTS guest os (VirtualBox running on OSX)

I don't have this problem in my VM with Eclipse Oxygen.3.

Anyone know how to fix this?

Regards, Ted.