JT pointed out that some documents on the Proofing page were not rendering correctly as PDFs; the process failed with an XSL-FO error. I investigated this, and found that the problem was caused by empty elements for the page numbers:

<biblScope type="start-page"></biblScope>
<biblScope type="end-page"></biblScope>

I started by confirming that this was the cause, by commenting them out (they're not needed anyway until the print publication finalizes the page numbers). Then I went back to fix the actual XSLT, and found the problem in the pdf_main_templates.xsl file, in the GetStartPage template. That's now doing some checking on the value of the tag before returning it, going from this:

<xsl:template name="GetStartPage">
  <xsl:param name="DefValue"/>
  <xsl:attribute name="initial-page-number">
    <xsl:choose>
      <xsl:when test="//teiHeader/fileDesc/sourceDesc/biblStruct/monogr/imprint/biblScope[@type='start-page']">
        <xsl:value-of select="//teiHeader/fileDesc/sourceDesc/biblStruct/monogr/imprint/biblScope[@type='start-page']" />
      </xsl:when>
      <xsl:otherwise>
        <xsl:value-of select="$DefValue" />
      </xsl:otherwise>
    </xsl:choose>
  </xsl:attribute>
</xsl:template>

to this:

<xsl:template name="GetStartPage">
  <xsl:param name="DefValue"/>
  <xsl:attribute name="initial-page-number">
    <xsl:choose>
      <xsl:when test="//teiHeader/fileDesc/sourceDesc/biblStruct/monogr/imprint/biblScope[@type='start-page'] and (string(number(//teiHeader/fileDesc/sourceDesc/biblStruct/monogr/imprint/biblScope[@type='start-page'])) != 'NaN')">
        <xsl:value-of select="//teiHeader/fileDesc/sourceDesc/biblStruct/monogr/imprint/biblScope[@type='start-page']" />
      </xsl:when>
      <xsl:otherwise>
        <xsl:value-of select="$DefValue" />
      </xsl:otherwise>
    </xsl:choose>
  </xsl:attribute>
</xsl:template>

I also noticed a couple of typos in the markup of a review, which I fixed, and reported to LN so he can fix them in his copy.