Voting not working: Getting AJAX related error

Voting doesn't work on my setup. I'm running it at http://teaquestions.org

This is a subsite of a WPMU/BP install.

The error console returns this each time a voting arrow is clicked:
GET http://questions.teatra.de/wp-admin/admin-ajax.php undefined (undefined)

The behavior is that nothing happens when the voting arrow is clicked.