Use german umlauts with a font where its not defined

Question

I need to use special font. There are no german umlauts defined. So everytime I get an error if I try to use them with my font (BentonSans):

\documentclass[a4paper,10pt]{article}
\pagenumbering{gobble}
\usepackage[absolute]{textpos}
%\usepackage[T1]{fontenc}
%\usepackage[utf8]{inputenc}
\usepackage{listings, mdframed}
\usepackage{color}
\usepackage{fontspec}
\usepackage{xunicode}
\usepackage{xltxtra}
\usepackage{setspace}
\usepackage{tabularx}
\usepackage{longtable}
\usepackage{ngerman}
\usepackage[ngerman]{babel}

\newfontfamily{\extralight}[UprightFont={* Extra Light}]{BentonSans}
\newfontfamily{\light}[UprightFont={* Light}]{BentonSans}
\newfontfamily{\thin}[UprightFont={* Thin}]{BentonSans}
\newfontfamily{\medium}[UprightFont={* Medium}]{BentonSans}

\definecolor{SPECIAL_GRAY}{HTML}{464854}

\begin{document}

\thin{äüöÄöüß}

\end{document}

Error:

$ xelatex main.tex
...
** WARNING ** "seac" character deprecated in Type 2 charstring.
** ERROR ** Type2 Charstring Parser: Parsing charstring failed: (status=-1, stack=5)

Output file removed.
 )
Error 256 (driver return code) generating output;
file main.pdf may not be valid.
Transcript written on main.log.

Is there any possibility to use this font and use umlauts? Currently I try with mathmode for which I set the font to use but if I have bold text somewhere or f.e. want to use \extralight my math font is still other one:

\usepackage{mathspec}
\setmathsfont(Digits,Latin,Greek)[Numbers={Lining,Proportional}, Path=fonts/]{BentonSans-Light.ttf}

Main aspect is that I have to migrate one document from MS Office to TeX and I must use this font. In MS Word it works just normally...

Answer 1

On the german list a similar (not the same) message was reported and there a workaround was to load the font by file name and to explicitly tell fontspec the extension:

\setmonofont{AnonymousPro-Regular}[Extension = .ttf]