Fatal error sending email with rules

The test uses node.uid.entity.mail.value, which has the effect of transforming the FieldItemList into the normal array of strings that is needed for the argument. Please try that. Also please take a look at 📌 Allow single-valued data selector input to be passed as an array for 'multiple' context fields Needs work , which might be a good usability improvement to make the input values less finicky.