Error 500 - probably .htaccess file problem

I have just installed a new ISPConfig 3 server under Debian wheezy. Everything works fine, but now I wanted to install a web shop software (called Shopware) but I alwyas get an error 500 when I try to call the install script. If I delete the .htaccess file I can access any file correctly through the webbrowser but with the .htaccess file active I always get the error 500. Below is the content of the .htaccess file:

P { margin-bottom: 0.21cm; }
<IfModule mod_rewrite.c>
RewriteEngine on

RewriteBase /shop/

# Https config for the backend
#RewriteCond %{HTTPS} !=on
#RewriteRule backend/(.*) https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

RewriteRule shopware.dll shopware.php
RewriteRule files/documents/.* engine [NC,L]
RewriteRule backend/media/(.*) media/$1 [NC,L]

RewriteCond %{REQUEST_URI} !(\/(engine|files|templates)\/)
RewriteCond %{REQUEST_URI} !(\/media\/(archive|banner|image|music|pdf|unknown|video)\/)
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ shopware.php [PT,L,QSA]

# Fix missing authorization-header on fast_cgi installations
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization},L]
</IfModule>

# Staging-Rules start
#SetEnvIf Host "staging.test.shopware.in" ENV=staging

DirectoryIndex index.html
DirectoryIndex index.php
DirectoryIndex shopware.php

# Disables download of configuration
<Files ~ "\.(tpl|yml|ini)$">
Deny from all
</Files>

# Enable gzip compression
<IfModule mod_deflate.c>
AddOutputFilterByType DEFLATE text/html text/xml text/plain text/css text/javascript application/json
</IfModule>

<IfModule mod_expires.c>
<Files ~ "\.(jpe?g|png|gif|css|js)$">
ExpiresActive on
ExpiresDefault "access plus 1 month"
FileETag None
<IfModule mod_headers.c>
Header append Cache-Control "public"
Header unset ETag
</IfModule>
</Files>
</IfModule>

# Disables auto directory index
<IfModule mod_autoindex.c>
Options -Indexes
</IfModule>

<IfModule mod_negotiation.c>
Options -MultiViews
</IfModule>

<IfModule mod_php5.c>
# php_value memory_limit 128M
# php_value max_execution_time 120
# php_value upload_max_filesize 20M
php_flag phar.readonly off
php_flag magic_quotes_gpc off
php_flag session.auto_start off
php_flag suhosin.session.cryptua off
php_flag zend.ze1_compatibility_mode off
</IfModule>

# AddType x-mapp-php5 .php
# AddHandler x-mapp-php5 .php