/usr/doc: can not build documentation: parser error : Failure to process entity chap.mirrors.ftp.index.inc

Last time I built the documentation was 2017. Since then my attempts to
build the /usr/doc failed. Just retrieving the recent /usr/doc via svn
(r52522), reinstalling textproc/docproj via portmaster -df (recursively
to ensure nothing is missing), ends up in errors. One of the prominent
error after performing a make distclean clean in /usr/doc is the error
shown below. What is wrong here?

[...]
env \
XML_CATALOG_FILES="file:///usr/obj/usr/doc/en_US.ISO8859-1/books/handbook/catalog-cwd.xml
file:///usr/doc/en_US.ISO8859-1/share/xml/catalog.xml
file:///usr/doc/share/xml/catalog.xml
file:///usr/local/share/xml/catalog" /usr/local/bin/xmllint --nonet
--noent --valid --dropdtd
--xinclude /usr/doc/en_US.ISO8859-1/books/handbook/book.xml >
book.parsed.xml.tmp warning: failed to load external entity
"/usr/doc/en_US.ISO8859-1/books/handbook/mirrors.xml.ftp.index.inc" /usr/doc/en_US.ISO8859-1/books/handbook/mirrors/chapter.xml:100:
parser error : Failure to process entity chap.mirrors.ftp.index.inc
&chap.mirrors.ftp.index.inc;
^ /usr/doc/en_US.ISO8859-1/books/handbook/mirrors/chapter.xml:100:
parser error : Entity 'chap.mirrors.ftp.index.inc' not defined
&chap.mirrors.ftp.index.inc;