woocommerce error in Hueman

Hi All.

I am using the huaman theme with Wooceommerce and, since a few days, encounter an error after a customer tries to checkout:

Fatal error: Call to undefined method WooCommerce::add_error() in /xxxx/mrgps.nl/wp-content/plugins/multisafepay/multisafepay.php on line 754

I have not installed new plugins etc neither have made major changes ofr updates. Also; the problem appears not to occur all the time…

Any suggestions highly appreciated!!

Thanks,
Hans