Solution: Manual Update
This error is caused by a fail WordPress version upgrade. Try a manual update.
- Download the latest WordPress zip file and unzip it.
- Back up your WordPress.
- Rename wp-includes and wp-admin directories to wp-includes.bak and wp-admin.bak.
- Upload the wp-includesand wp-admin directories from unzipped folder to your web host via FTP.
- Upload files from new wp-content to your existing wp-content folder. Replace and overwrite existing files.
- Upload the new version of the rest of files from the root directory to your existing WordPress root directory.
- Remove .maintenance at your WordPress directory via FTP.
- Login to WordPress admin and you will be given a link to URL like “http://domain.com/wordpress/wp-admin/upgrade.php”. Follow the link and instructions.
- Clear your cache to see the changes if you have caching enabled.
For more details about WordPress manual update, read http://codex.wordpress.org/Updating_WordPress#Manual_Update.