Editors unable to create version stacks

Our editor is having trouble uploading version stacks to Shade.
They have tried using a guest account and a shareable editor link.

While they can upload, add, and create folders, the stacking files won’t work, manual stacking fails as well.

It works for me in a private window with no full account login.

I’ve asked them to try a different browser and the Shade app, but they still can’t stack files.

Attached are the error messages.