WC redirects to error page after purchase

I am using the Divi theme from Elegant Themes and the latest version of WooCommerce 2.3.8.
After purchase, my customers get a white page with the following message:

{“result”:”failure”,”messages”:”
\n\t\t\t
We were unable to process your order, please try again.<\/li>\n\t<\/ul>\n”,”refresh”:”true”,”reload”:”false”}

Everything else works though – customers receive emails and the order shows as received on the backend.
I can’t seem to fix this, despite trying what feels like everything! I have of course re-installed WC and disabled plugins, to no avail.

Any ideas out there??

//Cissi

https://www.ads-software.com/plugins/woocommerce/