Versuch mal in der Apache COnfig das MultiViews zu deaktivieren und das "Require" Flag mit reinzunehmen.
Das "-MultiViews" soll wohl n Apache Bug sein.

<Directory /var/www/>
Options -Indexes -MultiViews
AllowOverride All
Require all granted
</Directory>
Und htaccess ohne Log und Anführungszeichen.
RewriteEngine On

RewriteRule "^admin/(.*).html$" "admin/index.php?controller=admin_$1" [QSA]
RewriteRule "^feed/(.*).xml$" "index.php?controller=feed_$1" [QSA]
RewriteRule "^page/(.*).html$" "index.php?controller=page_$1" [QSA]
RewriteRule "^download/([0-9]+)/.*.html$" "index.php?controller=page_category&category_id=$1 " [QSA]
RewriteRule "^download/[^/]*/([0-9]+)/.*.html$" "index.php?controller=page_category&subcategory_id = $1" [QSA]
RewriteRule "^download/[^/]*/[^/]*/([0-9]+)/.*.html$" "index.php?controller=page_article&id=$1" [QSA]
RewriteRule "^search.html$" "index.php?controller=page_category" [QSA]
RewriteRule "^user/(.*).html$" "admin/index.php?controller=admin_$1" [QSA]