Error al recuperar los datos del servidor intermedio

[Akronix]

Al intentar firmar en Ubuntu tanto desde Google Chromo como desde Firefox me salta este error:

javax.net.ssl.SSLHandshakeException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
      at java.base/sun.security.ssl.Alert.createSSLException(Alert.java:130)
      at java.base/sun.security.ssl.TransportContext.fatal(TransportContext.java:383)
      at java.base/sun.security.ssl.TransportContext.fatal(TransportContext.java:326)
      at java.base/sun.security.ssl.TransportContext.fatal(TransportContext.java:321)
      at java.base/sun.security.ssl.CertificateMessage$T12CertificateConsumer.checkServerCerts(CertificateMessage.java:647)
      at java.base/sun.security.ssl.CertificateMessage$T12CertificateConsumer.onCertificate(CertificateMessage.java:467)
      at java.base/sun.security.ssl.CertificateMessage$T12CertificateConsumer.consume(CertificateMessage.java:363)
      at java.base/sun.security.ssl.SSLHandshake.consume(SSLHandshake.java:393)
      at java.base/sun.security.ssl.HandshakeContext.dispatch(HandshakeContext.java:476)
      at java.base/sun.security.ssl.HandshakeContext.dispatch(HandshakeContext.java:447)
      at java.base/sun.security.ssl.TransportContext.dispatch(TransportContext.java:206)
      at java.base/sun.security.ssl.SSLTransport.decode(SSLTransport.java:172)
      at java.base/sun.security.ssl.SSLSocketImpl.decode(SSLSocketImpl.java:1506)
      at java.base/sun.security.ssl.SSLSocketImpl.readHandshakeRecord(SSLSocketImpl.java:1421)
      at java.base/sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:455)
      at java.base/sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:426)
      at java.base/sun.net.www.protocol.https.HttpsClient.afterConnect(HttpsClient.java:586)
      at java.base/sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:187)
      at java.base/sun.net.www.protocol.http.HttpURLConnection.getOutputStream0(HttpURLConnection.java:1461)
      at java.base/sun.net.www.protocol.http.HttpURLConnection.getOutputStream(HttpURLConnection.java:1432)
      at java.base/sun.net.www.protocol.https.HttpsURLConnectionImpl.getOutputStream(HttpsURLConnectionImpl.java:219)
      at es.gob.afirma.core.misc.http.UrlHttpManagerImpl.readUrl(UrlHttpManagerImpl.java:254)
      at es.gob.afirma.core.misc.http.UrlHttpManagerImpl.readUrl(UrlHttpManagerImpl.java:128)
      at es.gob.afirma.core.misc.http.UrlHttpManagerImpl.readUrl(UrlHttpManagerImpl.java:99)
      at es.gob.afirma.standalone.HttpManager.readUrl(HttpManager.java:37)
      at es.gob.afirma.standalone.protocol.IntermediateServerUtil.send(IntermediateServerUtil.java:82)
      at es.gob.afirma.standalone.protocol.IntermediateServerUtil.retrieveData(IntermediateServerUtil.java:72)
      at es.gob.afirma.standalone.protocol.ProtocolInvocationLauncherUtil.getDataFromRetrieveServlet(ProtocolInvocationLauncherUtil.java:64)
      at es.gob.afirma.standalone.protocol.ProtocolInvocationLauncher.launch(ProtocolInvocationLauncher.java:665)
      at es.gob.afirma.standalone.protocol.ProtocolInvocationLauncher.launch(ProtocolInvocationLauncher.java:138)
      at es.gob.afirma.standalone.SimpleAfirma.main(SimpleAfirma.java:691)
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 java.base/sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:388)
      at java.base/sun.security.validator.PKIXValidator.engineValidate(PKIXValidator.java:271)
      at java.base/sun.security.validator.Validator.validate(Validator.java:256)
      at java.base/sun.security.ssl.X509TrustManagerImpl.checkTrusted(X509TrustManagerImpl.java:230)
      at java.base/sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:132)
      at java.base/sun.security.ssl.CertificateMessage$T12CertificateConsumer.checkServerCerts(CertificateMessage.java:631)
      ... 26 more
Caused by: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
      at java.base/sun.security.provider.certpath.SunCertPathBuilder.build(SunCertPathBuilder.java:148)
      at java.base/sun.security.provider.certpath.SunCertPathBuilder.engineBuild(SunCertPathBuilder.java:129)
      at java.base/java.security.cert.CertPathBuilder.build(CertPathBuilder.java:297)
      at java.base/sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:383)
      ... 31 more

[Akronix]

Creo que esto lo puede arreglar: #321 (comment)