Listings in Latex with UTF-8 (or at least german umlauts)

Trying to include a source-file into my latex document using the listings package, i got problems with german umlauts inside of the comments in the code.Using

\lstset{extendedchars=\true,inputencoding=utf8x}

Umlauts in the source files (encoded in UTF-8 without BOM) are processed, but they are somehow moved to the beginning of the word they are contained in.So

// die Größe muss berücksichtigt werden

in the input source file, becomes

// die ößGre muss übercksichtigt werden

in the output file.

NOTE: since i found errors in my initial setup, i heavily edited this question