DKRed Error - No Board Outline - KiCAD 9

I’ve designed a pcb in KiCAD and am trying to order it from DKRed. Regardless of whether I upload the .kicad_pcb file or a gerber zip, I get a “Missing Outline File: The compressed file you submitted is missing an outline file. Your PCB cannot be built without an outline file.” I’m not even given the option to assign layers or view the pcb, it just kicks me back out to the upload screen. There is definitely an Edge_Cuts layer in my PCB, containing a complete, contiguous, rectangular board outline. The board shows up fine in the Kicad 3d viewer.

Weird: I have another PCB that I designed in a previous version of KiCAD. When I reopen that PCB in Kicad 9 and regenerate the Gerbers (using the same settings as the new PCB), DKRed accepts it without errors and shows a valid board preview.

Even weirder: I then tried creating a minimal test PCB: Brand new project, with nothing but a rectangle on the edge cuts layer and a single via. I tried uploading this, and got the same board outline error again.

Any ideas? I’m stuck and can’t use DKRed for any new projects as long as I’m getting this error.

Plot settings:

I’ve reached out via private message to get your ZIP archive to see if I can replicate this issue and provide feedback.