3178386 - Transactions fail in SAP Integration Suite, Managed Gateway for Spend Management and SAP Business Network Migration Guide with Error "Service Unavailable"

Transactional documents sent from SAP Integration Suite, Managed Gateway for Spend Management and SAP Business Network Migration Guide (CIG) fail in Transaction Tracker with either of the below errors :

Error: Service Unavailable

or

Error: Could not Send Message

or

Error: org.apache.cxf.transport.http.HTTPException: HTTP response '503: Service Unavailable' when communicating with http://<Domain Name>:<Port>/sap/bc/srt/<service name>

The service name indicates the service being triggered at runtime in ERP. It varies depending on the interface.
E.g. sap/bc/srt/xip/arba/PurchaseOrderExportRequest/<Client ID>, IDoc?sap-client=<Client ID>

The Domain Name, Port and Client ID are from the Connection page of your Managed Gateway for Spend&Network Project.

503: Service Unavailable, Cloud Connector Configuration, Could not Send Message, Service Unavailable, HTTP response 503 , CIG-PLT-00671, tenant, CIG , KBA , BNS-ARI-CI-FND-PRT , Managed Gateway for Managed Gateway Portal , BNS-ARI-SS , Strategic Sourcing , BNS-ARI-CI-SCC-BUY , Managed Gateway for Buyer Business Network SCC , BNS-ARI-CI-SRC , Managed Gateway for Sourcing Integration , BNS-ARI-CI-BUY , Managed Gateway for Procurement , BNS-ARI-CI-AN , Managed Gateway for Business Network , Problem