My email template shows a blank email or “an error has occurred”. How do you fix it?

Magento 2.4.x silently fails an email when a template has invalid directives: adjacent }}{{depend, a nested {{depend X}}...{{depend Y}}...{{/depend}}, or a {{var X}} that isn’t in the template’s @vars whitelist. The result is a blank email or the “we are sorry, an error has occurred” message. We pre-compute conditionals in PHP where needed, fix the directives, and re-test the render so the template produces correct, complete HTML.