The draft that is not opening could you provide a screenshot from the invoices screen showing the draft/invoice number so we can look into this further.
The only reason usually for this is possible corruption in browser on the url it directs to when you open up the draft. We have checked in a background tool and it opens the draft.Can we ask you to try l in a private/incognito window going to https://uk.selfemployed.intuit.com and signing in to QBSE and try viewing/editing the draft?
What browser are you currently using the view the draft invoice? Is it possible to test this in another browser or on a different device? As Emma suggested this is most likely a cache-specific issue with the URL for this invoice on your browser.