Right, I’ve managed to solve the problem. Maybe this answer helps other people who encounter the issue again. The problem is inherently in the installation procedure when installing via the package manager in ubuntu.
The actual issue was in the database configuration, the installation had set WP_CONTENT_DIR to /srv/www/wp-content/localhost, a directory I did not even know existed.
msvalkon@Lunkwill:/var/www/wordpress$ sudo cat /etc/wordpress/config-localhost.php
<?php
# Created by /usr/share/doc/wordpress/examples/setup-mysql
--- SNIP ----
define('WP_CONTENT_DIR', '/srv/www/wp-content/localhost');
define('WP_DEBUG', true);
?>
Apparently the .deb package creates the following directory structures:
/usr/share/wordpress <-- contains a wp-content-folder and symlinks to
/var/lib/wordpress/wp-content/uploads and /blogs.dir
/var/lib/wordpress <-- contains a wp-content-folder
/srv/www/wordpress <-- also contains a wp-content-folder
Modifying the permissions of /srv/www/wordpress/wp-content finally worked.