some non-ascii characters in the bibtex file, for example the long hyphen −, result in an error message like
! Package inputenc Error: Unicode char \u8:− not set up for use with LaTeX.
in order to find non-ascii characters in the bibtex file, the following command lists all these characters which can then be searched with a text editor:
tr -d "\000-\011\013-\177" < file.bib | sed '/^$/d'