XeTeX: “Failed to convert input string to UTF16″

http://workbench.haefelinger.it/archives/148

XeTeX allows me to hack away directly in Unicode. But what’s even more cool is that I can just use the fonts installed on my system without diving into some of TeX most darkest corners. However, when I compiled a larger document with xelatex, I faced some strange warnings like

This is XeTeX, Version 3.1415926-2.2-0.9995.2 (TeX Live 2009)
entering extended mode
[..]
** WARNING ** Failed to convert input string to UTF16...

and I had no explanation where this might come from. After a divide-and-conquerdebugging session, it turned out that my input was just fine. It was rather a setting that caused this warning:

\hypersetup{
     ..
     unicode=true          %  causes UTF-16 warning
}

After having removed that line, all UTF-16 warnings are gone.