Angular7 application cannot reach API once published

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I have a Angular 7 Application with .Net core 2.1, once the application is publish it cannot reach the API, which means no calls can be made, but before i published the application everything worked fine.
What could be the problem and how can i fix this?

Example of a call in my code that checks if the API is accessible:
(this is the first call in my application)

let availabilityHTTPCall = this.http.get("/v1/CheckAvailability");
    availabilityHTTPCall.subscribe(
      (result: boolean) => {
        this._webApiUnavalable = result;
        this.wizardOpen = true;
        console.log("HTTP Call returned data without error!", result);
      },
      (error) => {
        console.log("HTTP Call returned an error!", error);
        this._webApiUnavalable = false;
      },
      () => {
        console.log("HTTP Call complete!");
        this.PlacesOfService();
        this.LoadPhCodes();
      }
    );

What the API looks like in the controller:

[HttpGet]
     [Route("CheckAvailability")]
     public Boolean CheckAvailability()
     {
         return true;
     }

The error i get from my call:

"Http failure during parsing for http://localhost/"

"Unexpected token < in JSON at position 0"

What I have tried:

i have tried to insert a hashtag in-front of my URL of my API call which did not work, since this worked for fixing the reloading of my pages once published.