3665224 - Migration Assessment raises HTTP 503 Service Unavailable error

From Migration Assessment you perceive, following is perceived:

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ES Repository connection test failed
Destination connection test failed. The error reason is: 503 Service Unavailable on HEAD request for "http://<host>:<port>/URI"
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Upon checking SAP Cloud Connector trace (ljs_trace.log / scc_core.trc), below is raised matching this particular error above.

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20xx-xx-xx xx:xx:xx,xxx +0200#INFO#com.sap.core.connectivity.tunnel.client.handshake.ClientProtocolHandshaker#tunnel-client-31-9#          #Sending handshake request for tunnelId: account:///<subaccount-ID> and host: connectivitytunnel.cf.us10.hana.ondemand.com:443
20xx-xx-xx xx:xx:xx,xxx +0200#ERROR#com.sap.core.connectivity.tunnel.core.handlers.TunnelClientErrorHandler#tunnel-client-31-9#          #Unexpected problem during tunnel processing in channel [id: 0xaa957342, L:/<local ip address>:<port> - R:connectivitytunnel.cf.us10.hana.ondemand.com/52.23.1.211:443]; Reason: SSLEngine closed already; Exception type: io.netty.handler.ssl.SslClosedEngineException
20xx-xx-xx xx:xx:xx,xxx +0200#ERROR#com.sap.core.connectivity.tunnel.core.handlers.TunnelClientErrorHandler#tunnel-client-31-9#          #Unexpected problem during tunnel processing in channel [id: 0xaa957342, L:/<local ip address>:<port> ! R:connectivitytunnel.cf.us10.hana.ondemand.com/52.23.1.211:443]; Reason: error 104 - Connection reset by peer; Exception type: java.io.IOException
20xx-xx-xx xx:xx:xx,xxx +0200#WARN#io.netty.util.concurrent.DefaultPromise#tunnel-client-31-9#          #An exception was thrown by com.sap.core.connectivity.spi.ssl.SSLHandshakeValidator.operationComplete()
java.util.concurrent.ExecutionException: java.nio.channels.ClosedChannelException
    at io.netty.util.concurrent.DefaultPromise.get(DefaultPromise.java:364)
    at com.sap.core.connectivity.spi.ssl.SSLHandshakeValidator.operationComplete(SSLHandshakeValidator.java:36)
    at io.netty.util.concurrent.DefaultPromise.notifyListener0(DefaultPromise.java:603)
Suppressed: io.netty.handler.ssl.StacklessSSLHandshakeException: Connection closed while SSL/TLS handshake was in progress
        at io.netty.handler.ssl.SslHandler.channelInactive(Unknown Source)
20xx-xx-xx xx:xx:xx,xxx +0200#ERROR#com.sap.core.connectivity.tunnel.client.application.ApplicationClientConnectTask#Thread-97#          #Unexpected exception while establishing application tunnel connection for tunnelId: account:///<subaccount-ID>
java.io.IOException: error 104 - Connection reset by peer
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• Migration Assessment;
• SAP Cloud Connector(SCC) release independent;
• SAP Business Technology Platform(BTP).

sap cloud connector, cc, scc, connector, BTP, business technology platform, subaccount, sub-account, sub account, cf.us10.hana.ondemand.com, us10. hana.ondemand, 104, refused, connectivitytunnel, connectivitynotifiction, connectivity.tunnel.client.handshake, certificate , KBA , BC-MID-SCC , SAP Cloud Connector On-Demand/On-Premise Connectivity , Problem