Super late to this party, but it seems that you will get this error if your input XML is UTF-16. I.e.<?xml version="1.0" encoding="utf-16"?>Once I changed it to utf-8 this issue went away. Using Acrobat XI Pro.
Are you sure you used LiveCycle to author these forms? The "import" feature you describe is usually when an existing PDF/AcroForm is to be converted to a XML Form.