Did you upgrade WordPress from the dashboard? If so, go for a manual upgrade of WordPress. Follow the below procedure:
1. Download the latest version of WordPress from here
2. Make a backup of all your WordPress files and MySQL database.
3. Delete the directories wp-admin and wp-includes.
4. Delete all files from the root of your website starting with "wp-" except the wp-config.php file.
5. Extract the downloaded version of WordPress and upload all files and directories except the wp-content directory and the wp-config.php file.
6. To perform the upgrade, navigate to http://yoursitename/wp-admin/upgrade.php and follow the on screen steps.
That should do the trick.