Hi @astevens,
Thank you for the link. it was not the same problem, but helped. In my case the editor (that was also a reviewer on the same submission) was not able to download files of two submissions (both assigned to).
At one submission I only assigned the submission to the editor again.
In second submission assignment was not enough, as the editor declined the review. In this case I noticed in the DB in the table review_assignments that the declined was 1 and I changed it to 0. After that it works.
Regards, Primož