Error with the version 1.5.1

Hello, I have updated today for the version 1.5.1 and when I place an order I´m getting this error and is not placing the order:

Fatal error: Call to a member function payment_complete() on a non-object in /home/cork1/public_html/wp-content/plugins/paymill/lib/integration/woocommerce.inc.php on line 297

The plugin is installed in http://www.corkway.com
We are using Woocommerce version: 2.0.20 and WordPress 3.8.

What can we do? This is very urgent.
Thanks for your time.

Best regards

http://ww.wp.xz.cn/plugins/paymill/