After upgrading WordPress on a client website, it happened that I wasn’t able to access the admin section. When I went to the wp_login page, I had a “Fatal error: Cannot redeclare check_password_reset_key()” error. Surfing on the Internet, a common suggestion was to disable all the plugins. To do so, just access your website directory using an FTP client and rename the “plugin” folder to “plugin-OLD” and check if the dashboard is then visible.
This solution didn’t work for me. What I did instead was to download this WordPress version: http://wordpress.org/nightly-builds/wordpress-3.7-latest.zip and uploading the wp-login.php and wp-includes/user.php files to my remote WordPress folder. This solved the issue!