The SendBoard bot adds a comment on the card when an email gets sent/received. When you open the view/reply window, we fetch all of the comments and link them to the emails. If the SendBoard bot does not have permission to add comments to the card, emails cannot be linked successfully and causes an error.
To change the commenting permissions in Trello navigate to your board settings in the menu on the right and click 'Commenting Permissions'. Ensure that 'Members' are selected.
Visit this Trello article for more help with updating commenting permissions.
If you are still having trouble after updating the commenting permissions on your board, please contact us at firstname.lastname@example.org