Hello,
It looks like the Tails signing key is missing.
$ curl
http://tails.boum.org/tails-signing.key
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>404 Not Found</title>
</head><body>
<h1>Not Found</h1>
<p>The requested URL /tails-signing.key was not found on this server.</p>
<hr>
<address>Apache/2.2.16 (Debian) Server at tails.boum.org Port 80</address>
</body></html>
Robert