Almost no 404s

I’ve got a problem with my blog, which makes me crazy.

If you call up the non-existing page
http://wuerzblog.de/2007/03/26/gibt-es-nicht
you get my 404-errorpage and and statuscode 404. Fine.

But if you call up the also non-existing page
http://wuerzblog.de/2007/03/26/gibt-es-nicht
you will see the mainpage of the blog and get a statuscode 200. Bad, very bad.

But why?

Here’s my .htaccess

# BEGIN FAlbum
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^fotos/?([^/]*)?/?([^/]*)?/?([^/]*)?/?([^/]*)?/?([^/]*)?/?([^/]*)?/?([^/]*)?/?([^/]*)?/?$ /wp-content/plugins/falbum/wp/album.php?$1=$2&$3=$4&$5=$6&$7=$8 [QSA,L]
</IfModule>
# END FAlbum

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

# END WordPress
ErrorDocument 404 /index.php?error=404

(I already deleted the FAlbum-part and/or the ErrorDocument-Line – no change.)

I’m using this permalink-stucture:
/%year%/%monthnum%/%day%/%postname%/

What else for interest? PHP 5.2.0, mod_rewrite on (so my provider told me).

Any kind of help is welcome. 🙂