I have contacted https://givewp.com/contact-us/, with a subject “I am reporting a bug or security issue”. I received very prompt solution to comment out lines 34-37 in give-tributes/templates/ecard/footer-default.php:
// @todo: eventually remove as plugin matures. give_tributes_v11_donor_message_compat($payment_id);
Thank you for your reply. https://givewp.com/priority-support/ is behind the paywall. As I am helping my client, and I do not have premium access to this plugin. Since, this is clearly a plugin error – is there a way that it can be reported without a premium account?