The proxy is fine, it's amiga.org that's giving the error message.
<!-- http://amiga.org/index.php : retrieved in 0.422s -->
<!--
HeadIFrameEliminator:Bytes in: 469, out: 469, took 0.000 s
DegradeXHTMLRewriter:Bytes in: 469, out: 543, took 0.000 s
LinkRewriter:Bytes in: 543, out: 543, took 0.000 s
MainNavigationRewriter:Bytes in: 543, out: 543, took 0.000 s
CommonBlockRewriter:Bytes in: 543, out: 562, took 0.000 s
CSSRewriter:Bytes in: 562, out: 562, took 0.000 s
JavascriptRewriter:Bytes in: 562, out: 562, took 0.000 s
FrontPageModulesRewriter:Bytes in: 562, out: 562, took 0.000 s
-->
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html><head>
<title>403 Forbidden [Old Browser Edition]</title>
</head><body bgcolor="#ffffff">
<h1>Forbidden</h1>
<p>You don't have permission to access /index.php
on this server.</p>
<p>Additionally, a 404 Not Found
error was encountered while trying to use an ErrorDocument to handle the request.</p>
<hr>
<address>Apache mod_fcgid/2.3.5 FrontPage/5.0.2.2635 mod_bwlimited/1.4 mod_auth_passthrough/2.1 Server at amiga.org Port 80</address>
</body></html>
<!-- http://amiga.org/index.php : processed in 0.001s -->
The comments in that source are generated by the proxy. My guess is that amiga.org no longer likes my webserver reading content *sniff*