You have the course set up as separate groups. I would guess that the teacher does not have the permission to view all groups and is not the member of a group.
Does the teacher have a different role in this course?
In fact, he has only an authenticated user role in System, and in my course, he's not exactly of the Teacher role, he has a custom permissions role, but this has the Teacher archetype. In both cases, he has the permission of gradereport/grader:view .
So is this special role only in your course? Does he have a different role in the courses where he can see the gradebook? If so, you need to just look through the permissions of the special role - it would about have to be in there somewhere...
But there are people with this same role that can view the table normally... and generally, when it is a permission error, appears a message of permission denied, but it's not the case. It's something specific to this user, but it's being too tough to discover what it is!
From your screenshot though, it appears that he does not have access to any groups because there is not one listed by the groups heading.
If you want, I would be happy to take a quick look - it has to be something really simple but I would need the teacher login and an admin login. PM me if you want.