I believe that you changed the mail template using the "Edit Template" functionality. If you did do that, you would observe that the variable name part in the template has been URL encoded to "% 24 % 7 B object.signup_url % 7 D" instead of ${object.signup_url}, that is to say, the variable could not be found and the object.get_signup_url() function was never called.

To fix it, click on the "Edit" button to the top-left corner of the interface just below the title "Templates /Odoo Enterprise Connection". Then, click the "<>" button on the tool bar abovethe edit window to show the html source code of the tempalte, and change the variable back to ${object.signup_url} from "% 24 % 7 B object.signup_url % 7 D" .