Sadly, your client does not supply a proper User-Agent, and is consequently excluded.
We have an inordinate number of problems with automated scripts which do not supply a User-Agent, and violate the automated access guidelines posted at arxiv.org -- hence we now exclude them all.
(In rare cases, we have found that accesses through proxy servers strip the User-Agent information. If this is the case, you need to contact the administrator of your proxy server to get it fixed.)
If you are using the PDF Plug-in, it has many bugs and is forbidden here due to problems it causes at the server end. You must confirm that you have disabled it before access can be restored.
In Netscape try
Edit -> Preferences -> Navigator -> Applications,
Portable Document Format and uncheck the plug-in box.
Or delete the pdf plugin dll file from the
directory and restart browser. Or for Acroread4/Explorer5 users,
go into Acroread's
File : Preferences : General : Web_Browser_Integration
and make sure the little box is unchecked.
Note to MacOSX users: There is a bug in the Acrobat reader which
causes it to make endless streams of requests after having
successfully downloaded the full pdf. Note that it is not necessary to
use Acrobat at all, since pdf's from here render as well or better
in the default Preview.app on MacOSX. If for some reason you think you
need to use Acrobat, go to Acrobat
Preferences -> Internet
and turn off the
"Allow speculative downloading in the background" option, which comes
(incorrectly) turned on by default, and whose behavior is quite broken.
If you believe this determination to be in error, see https://arxiv.org/denied.html for additional information.