error with multisite

Hi I installed multisite and it seemed like it was working. But whebn I try to switch themes on one of the sub sites I get this error
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OK

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, [no address given] and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.
HTTP/1.1 200 OK Date: Sun, 29 May 2011 08:51:24 GMT Server: Apache/2.2.17 (Unix) FrontPage/5.0.2.2635 X-Powered-By: PHP/5.2.17 Expires: Wed, 11 Jan 1984 05:00:00 GMT Cache-Control: no-cache, must-revalidate, max-age=0 Pragma: no-cache Set-Cookie: _icl_current_language=en; expires=Mon, 30-May-2011 08:51:25 GMT; path=/ Last-Modified: Sun, 29 May 2011 08:51:25 GMT Keep-Alive: timeout=5, max=99 Connection: Keep-Alive Transfer-Encoding: chunked Content-Type: text/html; charset=UTF-8 757a
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Any suggestions?

This is what i Have in my .htaccess file

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule  ^[_0-9a-zA-Z-]+/(wp-(content|admin|includes).*) $1 [L]
RewriteRule  ^[_0-9a-zA-Z-]+/(.*\.php)$ $1 [L]
RewriteRule . index.php [L]
</IfModule>

# END WordPress

Any ideas??