/network/settings.php gives 404 error

I posted this several months ago and never figured out an answer. It’s now at crisis point and I’m hoping someone can help.

I can’t access /wp-admin/network/settings.php — I get a 404 error.

This, of course, disables:
– Network Settings
– Domain Mapping
– Domains

Before you ask…

1. YES, settings.php is there.
2. YES, it’s the latest one from the repository.
3. YES, the permissions are set to 644, same as everything else.
4. YES, I’ve checked .htaccess:

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^files/(.+) wp-includes/ms-files.php?file=$1 [L]

# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]

I’m completely stumped.