slave's cannot connect to jenkins servers with ECDH* SSL configurations via https
Replacing the bc* libraries in war/WEB-INF/lib/ bcprov-jdk15on-1.47.jar with bcprov-jdk15on-152.jar allows all connections to work with and without ECDH connections.
Upgrading the instance identity module (https://github.com/jenkinsci/instance-identity-module) to bouncycastle 1.52 should resolve this I think, I'm not aware of any other things that include bcprov in core.
Additionally, ssh-agent-plugin has a patch to fix the same issue for that plugin:
https://github.com/jenkinsci/ssh-agent-plugin/pull/8
And below is an example error of such a failed connection, although not specifically with a slave.
Caught exception while notifying Stash with id 695c0a35657a11c973a904ff993cd873b7283e1b
javax.net.ssl.SSLHandshakeException: Could not generate secret
at sun.security.ssl.ECDHCrypt.getAgreedSecret(ECDHCrypt.java:99)
at sun.security.ssl.ClientHandshaker.serverHelloDone(ClientHandshaker.java:1045)
at sun.security.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:348)
at sun.security.ssl.Handshaker.processLoop(Handshaker.java:979)
at sun.security.ssl.Handshaker.process_record(Handshaker.java:914)
at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:1050)
at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1363)
at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1391)
at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1375)
at org.apache.http.conn.ssl.SSLConnectionSocketFactory.createLayeredSocket(SSLConnectionSocketFactory.java:394)
at org.apache.http.conn.ssl.SSLConnectionSocketFactory.connectSocket(SSLConnectionSocketFactory.java:353)
at org.apache.http.impl.conn.DefaultHttpClientConnectionOperator.connect(DefaultHttpClientConnectionOperator.java:134)
at org.apache.http.impl.conn.BasicHttpClientConnectionManager.connect(BasicHttpClientConnectionManager.java:338)
at org.apache.http.impl.execchain.MainClientExec.establishRoute(MainClientExec.java:380)
at org.apache.http.impl.execchain.MainClientExec.execute(MainClientExec.java:236)
at org.apache.http.impl.execchain.ProtocolExec.execute(ProtocolExec.java:184)
at org.apache.http.impl.execchain.RetryExec.execute(RetryExec.java:88)
at org.apache.http.impl.execchain.RedirectExec.execute(RedirectExec.java:110)
at org.apache.http.impl.client.InternalHttpClient.doExecute(InternalHttpClient.java:184)
at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:82)
at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:107)
at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:55)
at org.jenkinsci.plugins.stashNotifier.StashNotifier.notifyStash(StashNotifier.java:556)
at org.jenkinsci.plugins.stashNotifier.StashNotifier.processJenkinsEvent(StashNotifier.java:207)
at org.jenkinsci.plugins.stashNotifier.StashNotifier.prebuild(StashNotifier.java:160)
at hudson.model.AbstractBuild$AbstractBuildExecution.preBuild(AbstractBuild.java:834)
at hudson.model.AbstractBuild$AbstractBuildExecution.preBuild(AbstractBuild.java:829)
at hudson.model.Build$BuildExecution.doRun(Build.java:144)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:537)
at hudson.model.Run.execute(Run.java:1741)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:98)
at hudson.model.Executor.run(Executor.java:381)
Caused by: java.security.InvalidKeyException: ECDH key agreement requires ECPublicKey for doPhase
at org.bouncycastle.jcajce.provider.asymmetric.ec.KeyAgreementSpi.engineDoPhase(Unknown Source)
at javax.crypto.KeyAgreement.doPhase(KeyAgreement.java:567)
at sun.security.ssl.ECDHCrypt.getAgreedSecret(ECDHCrypt.java:96)
... 32 more