ok ho risolto.

prima ho aggiunto una libreria (usando maven):
codice:
        <dependency>
            <groupId>org.bouncycastle</groupId>
            <artifactId>bcprov-jdk15on</artifactId>
            <version>1.58</version>
        </dependency>
poi, prima di fare qualsiasi tipo di connessione:
codice:
Security.insertProviderAt(new BouncyCastleProvider(), 1);
questo il trace dell'errore (che adesso non si presenta più):
codice:
javax.net.ssl.SSLException: java.lang.RuntimeException: Could not generate DH keypair
    at sun.security.ssl.Alerts.getSSLException(Alerts.java:208)
    at sun.security.ssl.SSLSocketImpl.fatal(SSLSocketImpl.java:1959)
    at sun.security.ssl.SSLSocketImpl.fatal(SSLSocketImpl.java:1916)
    at sun.security.ssl.SSLSocketImpl.handleException(SSLSocketImpl.java:1899)
    at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1420)
    at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1397)
    at sun.net.www.protocol.https.HttpsClient.afterConnect(HttpsClient.java:559)
    at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:185)
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1546)
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1474)
    at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:254)
    at org.apache.commons.io.FileUtils.copyURLToFile(FileUtils.java:1506)
    at com.mp.ydl.InstallYDL.install(InstallYDL.java:39)
    at com.mp.ydl.MainWindow$3$1.doInBackground(MainWindow.java:69)
    at com.mp.ydl.MainWindow$3$1.doInBackground(MainWindow.java:64)
    at javax.swing.SwingWorker$1.call(SwingWorker.java:295)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at javax.swing.SwingWorker.run(SwingWorker.java:334)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.RuntimeException: Could not generate DH keypair
    at sun.security.ssl.DHCrypt.<init>(DHCrypt.java:142)
    at sun.security.ssl.DHCrypt.<init>(DHCrypt.java:114)
    at sun.security.ssl.ClientHandshaker.serverKeyExchange(ClientHandshaker.java:711)
    at sun.security.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:268)
    at sun.security.ssl.Handshaker.processLoop(Handshaker.java:1026)
    at sun.security.ssl.Handshaker.process_record(Handshaker.java:961)
    at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:1072)
    at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1385)
    at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1413)
    ... 16 more
Caused by: java.security.InvalidAlgorithmParameterException: DH key size must be multiple of 64, and can only range from 512 to 2048 (inclusive). The specific key size 4096 is not supported
    at com.sun.crypto.provider.DHKeyPairGenerator.initialize(DHKeyPairGenerator.java:128)
    at java.security.KeyPairGenerator$Delegate.initialize(KeyPairGenerator.java:674)
    at sun.security.ssl.DHCrypt.<init>(DHCrypt.java:128)
    ... 24 more
spero possa servire a qualcuno!