If you are entering Fastly CDN keys on Upsun and end up with a 404 error, here's how to fix it:

• In your browser, with the 404 error active, open Inspect mode
• Open the Storage tab 
• Inside Storage, open Local Storage
• Locate the Fastly item (it may be called fast.upsun or similar), right click, and delete

You should then have a new, open form for re-entering your keys