Css background-images can be defined with data-urls. Most of the time these are base64 encoded, but there are cases where they are url-escaped.
One such example is the inline svg output by the svg-gradient function in LESS, which returns an inline url-encoded svg which itself contains a fill="url(#gradient)" attribute at some point. (see: http://lesscss.org/functions/#misc-functions-svg-gradient)
Currently Enyo Minifier's concatCss function does not correctly extract data-urls and breaks on such nested url() statements.