@pfefferle yes, that might be possible. BOX is strongly biased with the conventions of GoActivitPub where actorIRI/outbox is most of the time a safe assumption to make.
But there are mechanisms to dereference the oubox correctly after loading an Actor object, and usually I default to that, the "just append outbox to the URL" logic is a last resort.
Do you think you can send an email to the GoActivityPub mailing list with details about your issue, so I can try to find where exactly this happens? TY
Just click new post on this page: https://lists.sr.ht/~mariusor/go-activitypub-dev