500 internal error when trying to change network settings

I just installed WP Multisite from scratch in a subfolder of my webspace using the WP codex and this very nice tutorial by Ipstenu. I followed the tutorial really thoroughly!

I installed HP in a subfolder and made it running from root following the this codex article. I checked permalinks, they were working just fine. Then I turned on Multisite (sub-directories).

Everything went well. Except that now I cannot reach the dasboard for network settings from my sites menu. I always get a internal 500 error. Instead, I have to type the correct url manually to get there. E.g. to log in I use mySite.com/wordpress/wp-admin and to go manually to network settings I use mySite.com/wordpress/wp-admin/network/. This works. But as soon as I try to go there from inside the admin panel by clicking My Sites -> Network Admin -> Dashboard, the url jumps to mySite.com/wp-admin/network/ and I get the 500 error. Same, if I try to change any network settings and save them.

What’s wrong?
Could it be the .htaccess file?

The one I have at root level contains the following:

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

# END WordPress

The second which resides in “mySite.com/wordpress/” looks as following:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /mySite.com/wordpress/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /mySite.com/wordpress/index.php [L]
</IfModule>

# END WordPress

Any help? Thanks!
The site is: http://zugehoerigkeitsbehoerde.de/