`ucp-mappings.xsl` sheet throws "Content is not allowed in prolog" error

The new ucp-mappings.xsl throws an error each time it is run. It thus doesn't produce an output and subsequent steps in the pipeline cannot execute.

Error on line 2 column 1 of bGreen_pt3-11UCPMAPPED.xhtml:
  SXXP0003: Error reported by XML parser: Content is not allowed in prolog.
org.xml.sax.SAXParseException; systemId: file:/Users/atheg/Desktop/crawler/header_promotion_strategies/XSweet-staging-5d67cce275b7f65a0345e2ad500c29a8da146b6a/scripts/../outputs/Green/bGreen_pt3-11UCPMAPPED.xhtml; lineNumber: 2; columnNumber: 1; Content is not allowed in prolog.

Sometimes this occurs on line 2 column 1; other times it occurs on line 4 column 1:

Error on line 4 column 1 of bGreen05-11UCPMAPPED.xhtml:

Attached is the bash output for a whole book.prolog_content_error.log