Found the problem.
AM-1.10 defines "builddir=." That leading dot gets appended to all the
languages (German just happens to be the first one, in the list, so it
causes an error).
The fix is not pretty simple.
Replace the existing "HTML=" line in /doc/manual/Makefile.am with:
"HTML = $(addprefix $(if $(builddir),$(builddir)/,), $(addsuffix
.html, $(basename $(notdir $(TEXI)))))"
Appears to work fine on both AM-1.9 and AM-1.10
Yes, for me (AM 1.10) it works fine.
There is a similar problem, when doing the pdf target (make pdf).
The solution is also identical:
PDF = $(addprefix $(if $(builddir),$(builddir)/,), $(addsuffix .pdf,
$(basename $(notdir $(PNG)))))
in doc/manual/images/Makefile.am