3671641 - Managed Gateway from CIG is failing with (HTTP 503 - The server is currently unavailable to handle the request)

While working with CIG Outbound connection flow, same is raising error below:

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Managed Gateway for Spend&Network has successfully reached your server, however it has responded with an error - HTTP 503 - The server is currently unavailable to handle the request due to a temporarily overloading or maintenance of the server.
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Upon checking SCC trace (ljs_trace.log / scc_core.trc), following is perceived:

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20xx-xx-xx xx:xx:xx,xxx +0000#INFO#com.sap.core.connectivity.tunnel.client.handshake.ClientProtocolHandshaker#tunnel-client-150-9#          #Sending handshake request for tunnelId: account:///<sub-account-ID>/<Location-ID> and host: connectivitytunnel.hana.ondemand.com:443

20xx-xx-xx xx:xx:xx,xxx +0000#WARN#io.netty.util.concurrent.DefaultPromise#tunnel-client-150-9#          #An exception was thrown by com.sap.core.connectivity.spi.ssl.SSLHandshakeValidator.operationComplete()
java.util.concurrent.ExecutionException: javax.net.ssl.SSLHandshakeException: General SSLEngine problem
    at io.netty.util.concurrent.DefaultPromise.get(DefaultPromise.java:351)
    at com.sap.core.connectivity.spi.ssl.SSLHandshakeValidator.operationComplete(SSLHandshakeValidator.java:36)

Caused by: javax.net.ssl.SSLHandshakeException: General SSLEngine problem
    at sun.security.ssl.Handshaker.checkThrown(Handshaker.java:1521)

Caused by: javax.net.ssl.SSLHandshakeException: General SSLEngine problem
    at sun.security.ssl.Alerts.getSSLException(Alerts.java:192)

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(PKIXValidator.java:397)
    at sun.security.validator.PKIXValidator.engineValidate(PKIXValidator.java:302)

Caused by: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
    at sun.security.provider.certpath.SunCertPathBuilder.build(SunCertPathBuilder.java:141)

20xx-xx-xx xx:xx:xx,xxx +0000#TRACE#com.sap.core.connectivity.spi.util.ChannelUtil#tunnel-client-150-9#          #Closing channel immediately [id: 0xb0f7e47c, L:/<Local-IP>:<Local-Port> - R:connectivitytunnel.hana.ondemand.com/155.56.210.84:443]
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• SAP Cloud Connector(SCC) release independent;
• SAP Business Technology Platform(BTP);
• SAP Ariba/CIG.

certificate, refresh, renew, firewall, router, PKIX, digicert, issuer, SCC, 2.16.0, 2.15.0, 2.15.1, 2.15.2, 2.14.2, 2.14.1. 2.14.0, 2.13.0, 2.12, 2.10, sub-account, subaccount, cloud foundry, Neo, BTP, connection, SSL Engine, SSLEngine, WebUI, Web UI, Issuer, Subject DN, firewall, router, certificate, sign, signed, signing, TLS, TLS-termination, TLS termination, SSL, inspection, DigiCert Global, root CA, CA, hana.ondemand.com, ondemand.com, Caused by: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target, sap cloud connector, connector, CC, SCC, HTTP 503, 503, unavailable, handle, outbound, managed gateway, Spend&Network , KBA , BC-MID-SCC , SAP Cloud Connector On-Demand/On-Premise Connectivity , BNS-ARI-CI-PLT-CON , Managed Gateway - Buy side Connectivity , Problem