JS
(quick guide & example)
console.log("Fetch start"); fetch("URL") .then(res => { console.log(res); return res.text(); }) .then(txt => console.log(txt)) .catch(err => console.error(err));
BEFORE FETCH - "Fetch start" is not even logged. Something is wrong with your pre-processing.
DURING FETCH - Use console.log (res.status) to get server HTTP response code. Should be 200 (OK).
DURING FETCH - Turn on server side error reporting, console.log(txt) will show whatever server responded; Including server side script error messages.
AFTER FETCH - Not "fetch error". But if not getting intended results, your processing is wrong somewhere.