This seems to happen every few months, but in different ways. Usually I can work around it (e.g. by pasting when editing an existing issue rather than the create issue dialog), but this time cannot get it to work anywhere.
Have already tried using Incognito Mode, to no avail. Chrome is up to date.
When looking in the DevTools, keeps getting a 401 Unauthorized response to the second call to "/upload/{guid}/chunks" i.e. the "PUT" (the first call uses the "options" method)

Edit: OMG, I can't even upload the image as a file! Same issue.
response from chunks calls:
{"error":{"code":"Unauthorized","title":"Authorization header is missing or invalid"}}