Cannot find (possible) markup error

Given a "legacy" DocBook 4.3 file, quite large (approx. 640 KB, composed
out of several XML files using XInclude).

This source file validates w/o any errors/warnings using xmllint.

Trying to generate XHTML with the DocBook XSL stylesheets V1.77.1
(*no* customization used, using xsltproc) shows some messages like:

   Request for title of element with no title: literal

(see common/titles.xsl "title.markup")

I looked all over the file, checking every <literal> element
but could not find any markup error (as well as xmllint!).

Any hints how to further track down the (possible) markup errors?

Michael