I stumbled across this error when testing my unfinished project (Sudoku Solver / Quality Assurance)
As I understand, because of server error - instead of JSON, HTML is being sent (as response). Error points to fetch() method.
No clue how to solve this 
api.js :
'use strict';
const SudokuSolver = require('../controllers/sudoku-solver.js');
module.exports = function (app) {
let solver = new SudokuSolver();
app.route('/api/check')
.post((req, res) => {
let puzzle = req.body.puzzle
let coordinate = req.body.coordinate
let rowCoor = coordinate[0]
let colCoor = coordinate[1]
let value = req.body.value
let conflict = [];
// check rows
if (solver.checkRowPlacement(puzzle, rowCoor, colCoor, value)) {
conflict.push("row")
}
// check columnns
if (solver.checkColPlacement(puzzle, rowCoor, colCoor, value)) {
conflict.push("column")
}
// check region
if (solver.checkRegionPlacement(puzzle, coordinate, value)){
conflict.push("region")
}
// generate response
if (conflict.length === 0) {
res.json({
valid: true
})
} else {
res.json({
valid: false,
conflict: conflict
})
}
});
app.route('/api/solve')
.post((req, res) => {
let puzzle = req.body.puzzle
let output = solver.validate(puzzle)
output
? res.json({error: output})
: res.json({error: "no error"})
});
};