Ron

Thanks for the suggestion, midhead7.

Re-saving the Amazon credentials (just clicking on the “Save settings” button without doing anything else), seems to have fixed the issue.

I’m having the same issue.
Please post the solution that was provided.

I decided to try upgrading to WP Download Manager Pro (rather than using the free edition), in the hopes that it would resolve the current bug I was seeing with the Amazon S3 paid add-on.
Doing this appears to have resolved the issue.
So the bug must be limited to the free edition of WP Download Manager.