3218619 - Unable to open connection to backend system: Connection refused: no further information

Connection to a backend on-premise system fails and in the SAP Cloud Connector traces (ljs_trace/scc_core.trc) the below exception is found:

+0200#ERROR#com.sap.core.connectivity.spi.processing.AbstractProtocolProcessor#tunnel-client-31-2#0xd831a4da#Unable to open connection to backend system: Connection refused: no further information: <hostname>/<ipAddress>:<port>
io.netty.channel.AbstractChannel$AnnotatedConnectException: Connection refused: no further information: <hostname>/<ipAddress>:<port>
io.netty.channel.AbstractChannel$AnnotatedConnectException: Connection refused: no further information: <hostname>/<ipAddress>:<port>
Caused by: java.net.ConnectException: Connection refused: no further information
 at sun.nio.ch.SocketChannelImpl.checkConnect(Native Method)
 at sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:733)

SAP Cloud Connector

Sent message of type 4 (error) over tunnel channel, CIG, stuck, document, ariba, SAC, SCC, ERP, ECC, OnPrem, Cloud to On-premise, Access Control, Connection test, connection refused, io.netty.channel.AbstractChannel$AnnotatedConnectException, java.net.ConnectException, no further information, refused, scc, SAP Cloud Connector, cc, ip, port, hostname, ipAddress, firewall, blocked,  , KBA , BC-MID-SCC , SAP Cloud Connector On-Demand/On-Premise Connectivity , Problem