To get a URL for the Flash Charts the Piwik_url is used to generate the Link using http_build_query assuming that the arg_seperator is set to "&" but in some enviroments it is set to "&" which produces broken URLs because you escape the URLs after the generation for the Flash Chart.
.htaccess to overwrite this settings
php_value arg_separator.output &
php_value arg_separator.input &
php_value arg_separator &
(In ) - fix #5643 when default separator is not &, we not force it in the URL builder.
thanks for your help mneuhaus