3403469 - SAP Cloud Connector : " java.net.UnknownHostException - Unable to open connection to backend system"

The on-premise system, which has been mapped in the "Access Control" of the SAP Cloud Connector (SCC), is shown as "Not Reachable". 

The following entries are seen in the ljs_trace.log:

YYYY-MM-DD HH:MM:SS,000 +0000#ERROR#com.sap.core.connectivity.spi.processing.AbstractProtocolProcessor#tunnel-client-86-1#0xac546f25#Unable to open connection to backend system: <internal_hostname>
java.net.UnknownHostException: <internal_hostname>
    at java.net.InetAddress.getAllByName0(InetAddress.java:1335)
    at java.net.InetAddress.getAllByName(InetAddress.java:1247)
    at java.net.InetAddress.getAllByName(InetAddress.java:1175)
    at java.net.InetAddress.getByName(InetAddress.java:1125)
    at io.netty.util.internal.SocketUtils$8.run(SocketUtils.java:156)
    at io.netty.util.internal.SocketUtils$8.run(SocketUtils.java:153)
    at java.security.AccessController.doPrivileged(Native Method)
    at io.netty.util.internal.SocketUtils.addressByName(SocketUtils.java:153)

Or

YYYY-MM-DD HH:MM:SS,000 +00000#ERROR#com.sap.scc.rt#AccessControl connection checker#          #Error when resolving local host <hostname> --> java.net.UnknownHostException: <hostname>: Name or service not known 

SAP Cloud Connector 

hostname, java.net.UnknownHostException, access control, mapping, virtual, internal, Not reachable, on-premise,  system, cloud, connector, availability,  , KBA , BC-MID-SCC , SAP Cloud Connector On-Demand/On-Premise Connectivity , Problem