This is caused by the lack of a character set being defined by the RSS feed. The Google Toolbar defaults to the RFC Standard of US-ASCII. The content contains a single-quote character that can only be included in US-ASCII content as a '\x97'... But DU sends the literal character, causing a parsing error. The character set really needs to be set to the ISO-8859-1 character-set.
This is the error message that a feed validator returns: Your feed appears to be encoded as "ISO-8859-1", but your server is reporting "US-ASCII"
Since DU uses an Apache Web Server, there is a simple fix....
Apache. This can be done via the AddCharset (
http://httpd.apache.org/docs/mod/mod_mime.html#addcharset) (Apache 1.3.10 and later) or AddType (
http://httpd.apache.org/docs/mod/mod_mime.html#addtype) directives, for directories or individual resources (files). With AddDefaultCharset (
http://httpd.apache.org/docs-2.0/mod/core.html#adddefaultcharset)(Apache 1.3.12 and later), it is possible to set the default 'charset' for a whole server. For more information, see the article on Setting 'charset' information in .htaccess (
http://www.w3.org/International/questions/qa-htaccess-charset).
So, to answer your question... No, the button is not broken, it is following the rules strictly, as defined in the RFC and it is DU that is breaking the rules.