As per your error log, you are trying to export an empty object.
Elements in a schema can be optional (
minOccurs=0) and/or nillable. When an empty value for an element is encountered, the server will check the schema to decide whether to exclude the element (optional) or send the element with a nil attribute (nillable). In case an empty value is encountered but the element is not optional or nillable, the server will throw an exception, which you need to handle yourself in the microflow. This will occur regardless of the XML validation settings. It is recommended to make sure the data that is being exported is valid according to the schema.
Perhaps this will help you find the issue.