This has been a problem on my end for at least the last ten releases.
If I want to generate HTML documentation, I have to "make web", let it
fail, "make web", let it fail again, etc. Eventually it'll finish cleanly.
I also noticed that make web fails if I have the MS Mincho font
installed on my Gentoo box. Since this font attempts to cover a great
portion of the defined Unicode universe, I suspect that Lilypond is
trying to use it for some symbol for which Emmentaler is the desired
font. Next time I generate the documentation, I'll re-install MS Mincho
first and send in a report if the build fails again.