All links on subfolder blog redirect to root

I’m building a multisite network at http://goblinart.pl
WP is installed in http://goblinart.pl/wordpress/
with blogs at http://goblinart.pl/mps/ and http://goblinart.pl/vg/

The main site of http://goblinart.pl/mps/ displays correctly – a list of posts and a sidebar.
my index contains a simple query to decide on what to display:

<?php if (is_single()) { ?>
<?php include('../wordpress/singlepost.php'); ?>
<?php } else { ?>
<?php include('../wordpress/loop.php'); ?>
<?php } ?>

However any link redirects to http://goblinart.pl/index.php
Something is obviously wrong with my htaccess. It looks like this:

Options +FollowSymLinks
ErrorDocument 404 /404.gif 

RewriteEngine on

RewriteRule ^df.php http://goblinart.pl/vg/df-eng.php [R=301,L]

RewriteCond %{HTTP_HOST} ^mayday.w.staszic.waw\.pl$ [NC]
RewriteRule ^(.*)$ http://goblinart.pl/$1 [R=301,L]

RewriteEngine on

RewriteBase /
RewriteRule ^index\.php$ - [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).*) wordpress/$2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ wordpress/$2 [L]
RewriteRule . index.php [L]

]

Can anyone spot the problem here?