Tried a bunch of DocBook 3.1 files out via 'db2html' and
got this error for every single one. It was not present in
"stylesheets-0.10-2" (a Cygnus rpm which "tracks 
Norm's 1.44 stylesheets"). It is present in the beta, though:

jade:/usr/lib/sgml/stylesheets/nwalsh-modular/html/../common/dbl1ja.dsl:369:26:E:
unknown character name "U-524D\U-306E\U-30DA\U-30FC\U-30B8"
jade:/usr/lib/sgml/stylesheets/nwalsh-modular/html/../common/dbl1ja.dsl:378:26:E:
unknown character name "U-6B21\U-306E\U-30DA\U-30FC\U-30B8"

I gather these two lines are for generating navigation stuff:
369:   (make sequence (literal "&nav-prev;")))
378:   (make sequence (literal "&nav-next;"))

I am attaching a file of what passes for docbook which generates
this here on pinstripe but not with the 6.2 docbook tools. 

Note: db2html works fine: it just generates these errors. I suppose
it may not work in Japanese, of course...