An existing SAP agent was functioning correctly, but now is not connecting to SAP. The only message seen in the agent log is
Release : 12.3
Component : AUTOMATION ENGINE
The customer was able to resolve the issue by renaming the original HOST object and creating a new HOST object with the original object name.
Simply deleting and re-creating the HOST object did not work as it seems there were still references to that name.
Renaming the original object updated all of the references.
Creating a new HOST object with the original name treated it as a new agent, and is now connecting with the SAP instance as expected.