SSL certificate unknown issue between main pod and elastic search pod in GKE

I have a main pod in Google Kubernetes Engine(GKE) which connects to elastic search pod 9300. It was working fine until few days back but it is throwing some issue. I am looking for someone who has experience working with GKE, GCP and fixing SSL certificates.

SSL Problem error:14094416:SSL routines:ssl3_read_bytes:sslv3 alert certificate unknown
javax.net.ssl.SSLHandshakeException: error:14094416:SSL routines:ssl3_read_bytes:sslv3 alert certificate unknown
at io.netty.handler.ssl.OpenSslEngine.shutdownWithError(OpenSslEngine.java:598)
at io.netty.handler.ssl.OpenSslEngine.sslReadErrorResult(OpenSslEngine.java:799)
at io.netty.handler.ssl.OpenSslEngine.unwrap(OpenSslEngine.java:754)
at io.netty.handler.ssl.OpenSslEngine.unwrap(OpenSslEngine.java:831)
at io.netty.handler.ssl.OpenSslEngine.unwrap(OpenSslEngine.java:874)
at org.jboss.netty.handler.ssl.SslHandler.unwrap(SslHandler.java:1219)
at org.jboss.netty.handler.ssl.SslHandler.decode(SslHandler.java:852)
at org.jboss.netty.handler.codec.frame.FrameDecoder.callDecode(FrameDecoder.java:425)
at org.jboss.netty.handler.codec.frame.FrameDecoder.messageReceived(FrameDecoder.java:303)
at org.jboss.netty.channel.SimpleChannelUpstreamHandler.handleUpstream(SimpleChannelUpstreamHandler.java:70)
at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:564)
at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:559)
at org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:268)
at org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:255)
at org.jboss.netty.channel.socket.nio.NioWorker.read(NioWorker.java:88)
at org.jboss.netty.channel.socket.nio.AbstractNioWorker.process(AbstractNioWorker.java:108)
at org.jboss.netty.channel.socket.nio.AbstractNioSelector.run(AbstractNioSelector.java:337)
at org.jboss.netty.channel.socket.nio.AbstractNioWorker.run(AbstractNioWorker.java:89)
at org.jboss.netty.channel.socket.nio.NioWorker.run(NioWorker.java:178)
at org.jboss.netty.util.ThreadRenamingRunnable.run(ThreadRenamingRunnable.java:108)
at org.jboss.netty.util.internal.DeadLockProofWorker$1.run(DeadLockProofWorker.java:42)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)