MEAN-Stack -- GET http://localhost:3000/api/test net::ERR_EMPTY_RESPONSE - node.js

Question

I am a newbie in MEAN-Stack developing and need your help.
When I try a http get/post request using the Angular httpClient, I keep getting the following error message ERR_EMPTY_RESPONSE.
I start calling my get method within my frontend folder test.service.ts:
getTests(){
this.http
.get<{message: string, tests: TestModel[]}>('http://localhost:3000/api/test')
.subscribe((_testData)=>{
console.log(_testData);
});
}
The above used TestModel is my data model used in frontend:
export interface TestModel {
id: string;
title: string; }
Within my backend folder, my folder structure looks like in the picture attached. folder structure backend
This is how my app.js looks like (i replaced line 12, but the connection to database works).
const express = require('express')
const mongoose = require("mongoose");
const bodyParser = require("body-parser");
const test = require("./routing/test");
const app = express();
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
mongoose
.connect(
x
)
.then(() => {
console.log("Connected to database!");
})
.catch(() => {
console.log("Connection failed!");
});
//https://www.javatpoint.com/cors-in-mean-stack
app.use((req, res, next) => {
res.setHeader("Access-Control-Allow-Origin", "*");
res.setHeader(
"Access-Control-Allow-Headers",
"Origin, X-Requested-With, Content-Type, Accept, Authorization");
res.setHeader("Access-Control-Allow-Methods","GET, POST, PUT, DELETE, OPTIONS");
next();
});
app.use("/api/test", test);
module.exports = app;
My test.js where i expect the error origin looks as follows:
const express = require("express");
const testModel = require('../models/testModel');
const router = express.Router();
router.get("",(req,res,next)=>{
const testRooms = [
{
id: "1",
title: "1"
},
{
id: "2",
title:"2"
},
]
return res.json("testMessage", testRooms);
});
module.exports = router;
And finally my backend data model:
const mongoose = require('mongoose');
const express = require("express");
const router = express.Router()
const TestSchema = mongoose.Schema({
id: {type: String, required: true, unique: true},
title: {type: String, required: true},
});
module.exports = mongoose.model('TestSchema', TestSchema);
My server.js (which refers to the app.js) should work correctly, therefore I haven't posted it here.
It seems like the http module is not able to access the node backend at all, since I get the same error message if I delete the method in the "test.js". First google result hinted that this might be a CORS issue, but I don't think that this is the case here.
I am quite lost :((
If I missed to upload or mention something, just let me know.
Thanks a lot for any help and a happy new year's eve :)