The errors mentioned above have been fixed.
I assigned all students not yet in a group to a group which I added to the grouping. So there are now no students in "default group" with inadequate permissions to comment on the submission that cannot be deleted (only Admin, Managers and Teachers not assigned to a group can now view this submission).
This removed the "you can't add comments" error for all with the exception of the student who had originally made the submission when not in a group. I logged in as another member of her group and uploaded their assignment. And the errors have gone.
My recommendation for Teachers is now: Stay away from "grouping for student groups" unless you are satisfied that your groups are in order and know what you are doing and why...