Angular 7 not sending HttpClient POST request – instead dying with "Unknown Error

I Really run out of ideas and appreciate every bit of advice πŸ™‚

Sending the request:

  let options = {
     'headers' : {
        'accept': 'application/json'
     }
  }
  let data = new FormData()
  this.http.post("/api/adverts/search",data, options)
  .subscribe(
     data => {
        console.log("success")
        console.log(data)
     }
  );

Angular is not sending the request:

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But instead giving me an error in the js console:

enter image description here

All this while other post request to the same api is working just fine.

Postman to the same url works just fine:

enter image description here

CORS headers are set by server globally and work just fine for the other successfull request:

enter image description here

Does anyone have the slightest idea?
Im quite desperate πŸ˜€

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Source Url Angular 7 not sending HttpClient POST request – instead dying with "Unknown Error