How can I determine why some emails to custom addresses result in Error?

Most emails coming into Email Routing are forwarded correctly, but emails from one sender show “Error” for Result in the Activity Log on the Overview tab. These emails are sent to a legitimate custom address; other emails to that address are forwarded correctly. For these emails, SPF status is “pass”, DMARC status is “fail”, and DKIM status is “neutral”. Emails with the same statuses from other addresses are still forwarded, so it doesn’t appear that SPF, DMARC, or DKIM are the reason for the “Error” result.

How can I determine why these particular emails have a Result of “Error”?