When I checked the source of my webpage, I saw the link of the website of the author is displayed like this :
<cite id="dsq-cite-xxx"> http://url.com <span ...>Author name</span> </cite>
I changed the "comment_author_url" function call by "get_comment_author_url" and it seems to work properly. My comment header is now :
<cite id="dsq-cite-XXX"> <a href="http://url.com" rel="nofollow">Author name</a> </cite>
I already solved my problem, but I wanted to share the solution with you.