I was trying to edit https://community.openstreetmap.org/t/demo-of-new-vector-style-for-osm/144551/95 but got the error “An error occurred: The requested URL or resource could not be found”. Initially I tried adding a link to https://github.com/SomeoneElseOSM/SomeoneElse-vector-web-display/issues/20 again, and a screenshot, and that generated the error.
Oddly, I can make the edit in stages - Add some text, then edit the text to contain a link, then add the screenshot.