X_STREAM_ALREADY_FLUSHED
Means
a response header was written after the stream flushed
Typical cause
setCookie(), deleteCookie() or setRedirect() ran inside a <Hole> on a render: 'stream' page, after the status and headers left with the first flush. The hole shows its error markup and the monitor hears it
Fix
load: async () => { setCookie(name, value); return data; } in defineRoute, never under a <Hole>