Symptom
When validating source XML against the schema, the following error message is received:
The element 'ELEMENT_NAME' has invalid child element 'ELEMENT_NAME'
Cause
This error occurs after a user defined field (UDF) is created in Salesforce, this change is published to the Salesforce API.
Resolution
Any change made to the Salesforce API is a breaking change and will require a refresh of the schema followed by re-selecting the object at step level.