The javascript in question sets a cookie by the name mistertango[collect][mt_fp]
.
When cookies was defined (RFC 6265, I guess) it seems they didn't really specify what characters you're allowed to use in a cookie name, other than basically «text».
This causes some problems with parsing cookie names. Django relies on Python's http.cookies for this, and it seems http.cookies doesn't allow brackets in cookie names. http.cookie failes to parse cookie pairs with brackets in it, and doesn't parse pairs after that which means it doesn't see the sessionid
cookie it uses for authentication.
I'm not able to tell if Django/http.cookie should or shouldn't support this.
PHP does however seem to support it (even if it's broken), while Ruby on Rails does not.
The easy solution is to use only alphanumeric characters in cookie names.
For your case, the best solution is to get the javascript author to change their cookie name. If that's not possible, or in the mean time, you could host the javascript yourself and change the cookie name in your copy. (This may not work if the cookie is used for something outside of this javascript snippet, but I don't really understand Javascript and does not see what it is used for.)