Error 'unable to find valid certification path to requested target' when accessing Site Export/Import in EPP 5.2 behind Apache with SSL

Issue

When using Apache with SSL in front of my EPP installation I can't access the Site Export/Import tool. It crashes with the following errors below errors in the log. The SSL is configured only in Apache, it isn't in EPP.

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

[org.exoplatform.application.gadget.GadgetImporter] Unable to retrieve metadata of url: https://{host}/rest-{portal-container}/jcr/repository/portal-system/production/app:gadgets/app:SiteExportImport/app:data/app:resources/Application.gadget.xml

If I reach EPP directly and bypass Apache, the gadget it works.