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
  at sun.security.ssl.Alerts.getSSLException(Unknown Source)
  at sun.security.ssl.SSLSocketImpl.fatal(Unknown Source)
  at sun.security.ssl.Handshaker.fatalSE(Unknown Source)
  at sun.security.ssl.Handshaker.fatalSE(Unknown Source)
  at sun.security.ssl.ClientHandshaker.serverCertificate(Unknown Source)
  at sun.security.ssl.ClientHandshaker.processMessage(Unknown Source)
  at sun.security.ssl.Handshaker.processLoop(Unknown Source)
  at sun.security.ssl.Handshaker.process_record(Unknown Source)
  at sun.security.ssl.SSLSocketImpl.readRecord(Unknown Source)
  at sun.security.ssl.SSLSocketImpl.performInitialHandshake(Unknown Source)
  at sun.security.ssl.SSLSocketImpl.startHandshake(Unknown Source)
  at sun.security.ssl.SSLSocketImpl.startHandshake(Unknown Source)
  at sun.net.www.protocol.https.HttpsClient.afterConnect(Unknown Source)
  at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(Unknown Source)
  at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(Unknown Source)
  at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown Source)
  at java.net.HttpURLConnection.getResponseCode(Unknown Source)
  at sun.net.www.protocol.https.HttpsURLConnectionImpl.getResponseCode(Unknown Source)
  at org.jabref.logic.net.URLDownload.openConnection(URLDownload.java:319)
  at org.jabref.logic.net.URLDownload.asInputStream(URLDownload.java:254)
  at org.jabref.gui.externalfiles.FileDownloadTask.call(FileDownloadTask.java:27)
  at org.jabref.gui.externalfiles.FileDownloadTask.call(FileDownloadTask.java:14)
  at org.jabref.gui.util.BackgroundTask$3.call(BackgroundTask.java:174)
  at org.jabref.gui.util.DefaultTaskExecutor$1.call(DefaultTaskExecutor.java:122)
  at javafx.concurrent.Task$TaskCallable.call(Task.java:1423)
  at java.util.concurrent.FutureTask.run(Unknown Source)
  at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
  at java.util.concurrent.FutureTask.run(Unknown Source)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
  at java.lang.Thread.run(Unknown Source)
Caused by: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
  at sun.security.validator.PKIXValidator.doBuild(Unknown Source)
  at sun.security.validator.PKIXValidator.engineValidate(Unknown Source)
  at sun.security.validator.Validator.validate(Unknown Source)
  at sun.security.ssl.X509TrustManagerImpl.validate(Unknown Source)
  at sun.security.ssl.X509TrustManagerImpl.checkTrusted(Unknown Source)
  at sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(Unknown Source)
  ... 27 more
Caused by: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
  at sun.security.provider.certpath.SunCertPathBuilder.build(Unknown Source)
  at sun.security.provider.certpath.SunCertPathBuilder.engineBuild(Unknown Source)
  at java.security.cert.CertPathBuilder.build(Unknown Source)
  ... 33 more