asciidoctor-pdf: PDF generation reports parse failure. then hangs - but HTML generates fine.

The source docs convert to HTML no problem with Asciidoctor 1.5.2. But when I run a PDF conversion, I get an error:

Failed to parse formatted text: The old quoting rules based on XML/ISO mnemonic patterns 
(<code>&ohmgr;</code> etc) are replaced by specifying ADL to be UTF-8 based, and any 
exceptions to this requiring ASCII encoding should use the <code>\Uhhhh</code> style of 
quoting unicode used in various programming languages.

Asciidoctor then appears to hang.

The command line was:

asciidoctor -a current_year=2015 \
    -a stylesdir=../../../spec-publish-asciidoc/resources/css   \
    -a resources_dir=../../../spec-publish-asciidoc/resources \
    -a base_dir=../../../specifications-BASE \
    -a uml_export_dir=../UML \
    -a pdf-style=openehr_full_pdf-theme.yml \
    -a pdf-stylesdir=../../../spec-publish-asciidoc/resources \
    -r asciidoctor-pdf -b pdf --out-file=ADL2.pdf --trace master.adoc

The .adoc source of that text is:

The old quoting rules based on XML/ISO mnemonic patterns (`&ohmgr;` etc) are replaced by specifying ADL to be UTF-8 based, and any exceptions to this requiring ASCII encoding should use the `\Uhhhh` style of quoting unicode used in various programming languages.