The online payment provider Klarna requires validation callbacks with a 200 HTTP reply with the latest version (3) of their payment solution. If the validation callback fails, the error "Sorry, this purchase cannot be processed." is displayed.
The Klarna validation callback triggers a ruleset in the default setup of Cloudflare Web Application Firewall (WAF). To solve this issue with Klarna v3, add the following Page Rule with Cloudflare:
Replace "domain.com" with your domain name and select "Disable Security" as setting for the Page Rule. Click "Save and deploy" and Klarna v3 should work again.
Note that this applies for users of WooCommerce. You may need to adjust the Page Rule for other e-commerce solutions with Klarna.