mod_rewrite prepending subdomain name to query string

I am trying to force a redirect from the www subdomain of a website to the forum subdomain, while preserving query string. In other words, I expect this:

http://www.website.com/index.php?key=value

To turn into:

http://forum.website.com/index.php?key=value

So I wrote this supposedly simple rule:

RewriteCond %{HTTP_HOST} ^www\.website\.com$
RewriteRule (.*) http://forum.website.com/$1 [L,QSA,R=301]

This is the only rewrite rule I have in my .htaccess.

The sample url above gets rewritten into:

http://forum.website.com/forum/index.php?key=value

What is this strange (and, to me, unexpected) behavior? What am I doing wrong / missing?

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EDIT 1: Added rewrite logs (trimmed useless date info)

[www.website.com/sid#7f3580c0a4d8][rid#7f3580d935开发者_JAVA技巧a0/initial] (3) [perdir /var/www/website/] strip per-dir prefix: /var/www/website/forum/test -> forum/test
[www.website.com/sid#7f3580c0a4d8][rid#7f3580d935a0/initial] (3) [perdir /var/www/website/] applying pattern '(.*)' to uri 'forum/test'
[www.website.com/sid#7f3580c0a4d8][rid#7f3580d935a0/initial] (4) [perdir /var/www/website/] RewriteCond: input='www.website.com' pattern='^www\.website\.com$' => matched
[www.website.com/sid#7f3580c0a4d8][rid#7f3580d935a0/initial] (2) [perdir /var/www/website/] rewrite 'forum/test' -> 'http://forum.website.com/forum/test'
[www.website.com/sid#7f3580c0a4d8][rid#7f3580d935a0/initial] (2) [perdir /var/www/website/] explicitly forcing redirect with http://forum.website.com/forum/test
[www.website.com/sid#7f3580c0a4d8][rid#7f3580d935a0/initial] (1) [perdir /var/www/website/] escaping http://forum.website.com/forum/test for redirect
[www.website.com/sid#7f3580c0a4d8][rid#7f3580d935a0/initial] (1) [perdir /var/www/website/] redirect to http://forum.website.com/forum/test [REDIRECT/301]
[forum.website.com/sid#7f3580c54d70][rid#7f3580d935a0/initial] (3) [perdir /var/www/website/] add path info postfix: /var/www/website/forum/forum -> /var/www/website/forum/forum/test
[forum.website.com/sid#7f3580c54d70][rid#7f3580d935a0/initial] (3) [perdir /var/www/website/] strip per-dir prefix: /var/www/website/forum/forum/test -> forum/forum/test
[forum.website.com/sid#7f3580c54d70][rid#7f3580d935a0/initial] (3) [perdir /var/www/website/] applying pattern '(.*)' to uri 'forum/forum/test'
[forum.website.com/sid#7f3580c54d70][rid#7f3580d935a0/initial] (4) [perdir /var/www/website/] RewriteCond: input='forum.website.com' pattern='^www\.website\.com$' => not-matched
[forum.website.com/sid#7f3580c54d70][rid#7f3580d935a0/initial] (1) [perdir /var/www/website/] pass through /var/www/website/forum/forum

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EDIT 2: This is my website.conf:

<VirtualHost *:80>
    ServerName website.com

    DocumentRoot /var/www/website/forum/
</VirtualHost>

<VirtualHost *:80>
    ServerName www.website.com

    DocumentRoot /var/www/website/forum
</VirtualHost>

<VirtualHost *:80>
    ServerName forum.website.com

    DocumentRoot /var/www/website/forum
    <Directory />
         AddHandler fcgid-script .php
         FCGIWrapper /usr/lib/cgi-bin/php5 .php
         Options ExecCGI Indexes FollowSymLinks MultiViews
         AllowOverride All
         Order allow,deny
         allow from all
        </Directory>
</VirtualHost>

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Checking your rewrite logs I see that forum/ is already part of URL. Try this instead:

RewriteCond %{HTTP_HOST} ^www\.website\.com$
RewriteRule ^forum/(.*) http://forum.website.com/$1 [L,QSA,R=301]