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 (<2.17)/scc_core.trc (>=2.17)):

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 

"Image/data in this KBA is from SAP internal systems, sample data, or demo systems. Any resemblance to real data is purely coincidental."

SAP Cloud Connector release independent.

hostname, java.net.UnknownHostException, access control, mapping, virtual, internal, Not reachable, on-premise,  system, cloud, connector, availability, cloud connector, administration ui, access control, system mapping, add system, check availability, not reachable, on-premise system, sap cloud connector, SCC, backend system, unknown host exception, hostname resolution, internal hostname, java.net.UnknownHostException, ljs_trace.log, scc_core.trc, connection checker, local host, name not known, DNS issue, hostname misspelling, network admin, backend connectivity, host resolution, cloud to on-premises, access control mapping, system unreachable, , KBA , BC-MID-SCC , SAP Cloud Connector On-Demand/On-Premise Connectivity , Problem