a few days ago, i was getting a 404 (not found) error when i go to my wp-admin. after searching the forum and google, i came across so many posts and suggestions, but none of them worked for me. i am enumerating them here, as it may work for some people searching for help on that error. here are the things i tried:
1. cleared my cache
2. re-uploaded wp-admin
3. re-uploaded wp-includes
4. disabled plugins (via ftp by renaming plugins folder, since i can't log in)
5. added code
/** wp-admin login fix */ @define('ADMIN_COOKIE_PATH', '/');
to my wp-config.php file as suggested here http://www.kinocreative.co.uk/hints-and-tips/wordpress-cant-login-to-wp-admin/
6. changed folder permissions as suggested here http://www.techzoomin.com/wordpress-login-issueswp-admin-showing-404-error-page/
7. checked my login file (among others) as suggested here http://codex.wordpress.org/Login_Trouble
i then discovered that i was able to log in using one of my other usernames that has editor role... but none of my admin usernames can log in. i went into phpmyadmin and replaced the wp_capabilities of my admin to make it the same as the editor -- and i was able to log in!
not sure if this is a bug... around that time, i had already sought the help of my hosting provider, and they replied that there was a symbol lookup error of suhosin. they had resolved the issue on their side, and i was able to log in to wp-admin again.
anyway i hope this post helps others looking for a solution to that problem, and i hope someone can also look into that issue i encountered (able to login as editor, but not as admin) -- if its really a bug, or just a hosting issue on my side.