Answer
Check for a custom enumeration value that was added to the database but never approved/published — a mismatch between how many values exist in the database for an enumeration and how many the client expects is a classic symptom of an enumeration change that was made directly in the database without going through the proper approve-and-publish step. Approving and publishing the pending enumeration change resolves the DB-vs-client value-count mismatch causing this error.