Why does Send Dunning Letters end with an error?

Summary:

Hi all,

We are planning to implement Dunning Letters for specific customers in Oracle Fusion 25D. Our goal is a "lean" setup—we want to use Dunning functionality without implementing the full Advanced Collections suite (strategies, scoring, etc.).

Steps Completed So Far:

1. Dunning Configuration: Created a new Staged Dunning Letter via Manage Dunning Configurations.
2. Set Assignment: Added the necessary Set Assignment via Manage Business Unit Set Assignment.
3. Customer Profile: Enabled "Send Dunning Letters" at the Customer Account level.
4. Testing: Ran the "Send Dunning Letters" ESS program for a customer with a confirmed past-due balance.

The ESS process ends in Error. No output is generated, and the log file is not providing a clear root cause.