Symptom: User is redirected to /wp-admin/upgrade.php?.. when trying to access wp-admin
Cause: This occurs when the version number in wp-include/version.php is different than the one in SQL's wp_options table db_version value.
1. Create a backup of the database and files.
2. Check the database value for "db_version" in the wp_options table.
3. Update $wp_db_version in /wp-includes/version.php to match the database version.