Checkbox unvisible for Comments

Just found this support-forum

The Checkbox is displayed for the contact-form
But it’s invisible in the Comments-Form.

There is an empty space between the website-edit-field and the Send-Button where the checkbox should be and the html-code for the checkbox is there (i checked the code of the site) – but it isn’t displayed/visible

Any idea/solution? (it works fine on another site i’ve tested – but this one has a different template…)

The page I need help with: [log in to see the link]