I think maybe it's the error in your update server. If you get update success message when you click 'Update to 1.8.1' button. You can ignore this message. We will fixed it as soon.
No, i get the suggest when I download updated theme from themeforest and deleting the existing theme and then putting the theme to my website trough FTP.
Catchable fatal error: Object of class WP_Error could not be converted to string in /var/www/virtual/are.arkku.net/weebs.fi/htdocs/wp-includes/formatting.php on line 1250