Unable to upload screenshots to G Suite Marketplace

I tried to upload a screenshot to publish a private Spreadsheet add-on.
However, it did not complete with a "Not saved" message.

"Not saved" message image

The screenshot field is a required and I cannot publish the add-on.

Steps to reproduce:

1. Click "Add screenshot"
2. Upload an image
3. Click “Done"

• The requirements for a screenshot are:

Screenshots of your application. At least one is required, but you can optionally provide up to five screenshots. These should be 1280 x 800 pixels in size. If necessary, screenshots 640 x 400 pixels or 2560 x 1600 pixels in size can be used. Your screenshots should have square corners and no padding (full bleed).

• After uploading a screenshot that fulfills the requirements you need to make sure that all other required fields (e.g. Developer website ) are filled out
• Once all required fields are filled out correctly, the Update button becomes enabled

• After clicking on the Update button your screenshot and other changes will become saved