Cannot Access wp-admin Backend

PROBLEM: I am locked out of my wp-admin backend and have been unable to gain access even after disabling addons.

EXPERIENCE: My site has a custom URL to access the admin backend. Whenever I attempt to reach the login page with either that custom URL, /wp-admin, or /wp-admin.php, I get the following 404 error:

Not Found

The requested URL /[CUSTOM URL SLUG] was not found on this server.
Apache/2.2.15 (Red Hat) Server at [MY SITE] Port 80

Attempts to access admin pages with /wp-admin, or /wp-admin.php produce similar results.

The problem began occurring after a large number of plugin upgrades at once via the interface.

STEPS TAKEN: I’ve tried changing the custom login URL in the database (wp_options), which I can still access, and restarted the server without any resulting difference. I’ve also tried disabling all plugins as well as manually reinstalling WP. No difference occurred.

I can’t figure out what else could be the issue or how else to troubleshoot.

Any suggestions are welcome.