This error commonly occurs when you've recently updated your org from an old version of Kulturra.
To fix this issue, please follow the steps below:
Click on the App Launcher and search for Payment Center Settings. Go to Payment Center Settings | Payment Receipt button:
On the page that comes up, click Edit Template:
Scroll all the way to the first line of the template and add the following:
markuprenderUsingSystemContextWithoutSharing="true"
For example:
markup<messaging:emailTemplate subject="{!Relatedto.fw1__Payment_Receipt_Email_Subject__c}" recipientType="Contact" relatedToType="fw1__Payment__c" renderUsingSystemContextWithoutSharing="true">
Save your template.
If you have two templates, please edit both of them.
For more information, please check this article:
Secure Guest User Record Access Guidelines - Full Version
If you followed the steps above and the issue still occurs, feel free to contact us here: https://kulturra.com/contact/