How do I troubleshoot errors with email triggers?
If an email template trigger exists, look at the email template. If the email template contains an attachment, test its validity by sending an email manually from the process record. If the attachment is still valid, the file will be visible in the Attachments section of your email. If the attachment is not visible in the Attachments section, then you know that this file is corrupt or no longer valid. Remove the reference to this file in the Attachments section of the email template then upload a new file.