Hello @user593537 ,
This is an error our engineers have seen occur specifically with Subject AB emails, when trying to access the specific email's reporting page. I've marked your account as affected, and submitted the case to the engineers for further review. If and when they have an update, they'll make sure to notify you.
In the meantime, you can access the email's reporting through the My Campaigns page. Just click its name, go to the Reporting tab, and select the bounces for whichever separate sendout you want to view.