Only the main blog works. Modrewrite problem?

Hey folks,

I have a new Multisite installation on my localhost and on the remote.
I chose the subfolder structure, added the needed lines to wp-config and htaccess.
the blog is located on my localhost, in a subfolder “IMM”.

but somehow, only my main blog works. if i add a new one, then the dashboard cannot be loaded – error 403.
also it does not load any css, or images if i visit the site.

The .htaccess looks like this:

RewriteEngine On
RewriteBase /imm/
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).*) D:/XAMPP/htdocs/IMM/$2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ D:/XAMPP/htdocs/IMM/$2 [L]
RewriteRule . index.php [L]

Options +Indexes

any idea, where i should start to troubleshoot?