Unable to post FI document; ECS document number '&1', ECS year '&2'

What causes this issue?

The system was unable to post the FI document for the following reasons:
Suspense accounting has been suppressed for this document.
An error occurred when the system attempted to post the document.

System Response

The system created ECS item &V1&.

How to fix this error?

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.