net::ERR_INVALID_HTTP_RESPONSE error after post request with Angular 7

I am working on small web application and I am using angular 7 and asp.net core web api as a back end. I am trying to make http post request with angular. My service returns string (token) and when I receive it, I want to show it in alert box.

I have tested my service with Postman and everything works as expected.

From the browser my request is successfully mapped to the controller's action method. I have set breakpoint in this method body and it successfully returns token without any problem.

But httpclient returns error:

[object Object]

And in the browser console I see this:

POST https://localhost:44385/api/auth net::ERR_INVALID_HTTP_RESPONSE

I have two methods (for POST and for GET) in a service class injected into component. They look like these:

logIn(username: string, password: string) {

    const encodedCredentials = btoa(username + ':' + password);

    const httpOptions = {
        headers: new HttpHeaders({
            "Authorization": "Basic " + encodedCredentials,
            "Access-Control-Allow-Origin":"*"
        }),
        responseType: 'text' as 'text'
    };

    this.http.post(this.urlBase + 'auth', null , httpOptions)
    .subscribe(result => {
        alert(result);
    }, error => {
        alert(error); // [object Object]
    });
}

get(){
    return this.http.get(this.urlBase + 'auth', {responseType: 'text'});
}

What can be problem?

Update:

HttpErrorResponse {headers: HttpHeaders, status: 0, statusText: "Unknown Error", url: null, ok: false, …}
error: ProgressEvent
bubbles: false
cancelBubble: false
cancelable: false
composed: false
currentTarget: XMLHttpRequest {__zone_symbol__xhrSync: false, __zone_symbol__xhrURL: "https://localhost:44385/api/auth", __zone_symbol__loadfalse: null, __zone_symbol__errorfalse: null, __zone_symbol__xhrListener: ƒ, …}
defaultPrevented: false
eventPhase: 0
isTrusted: true
lengthComputable: false
loaded: 0
path: []
returnValue: true
srcElement: XMLHttpRequest {__zone_symbol__xhrSync: false, __zone_symbol__xhrURL: "https://localhost:44385/api/auth", __zone_symbol__loadfalse: null, __zone_symbol__errorfalse: null, __zone_symbol__xhrListener: ƒ, …}
target: XMLHttpRequest {__zone_symbol__xhrSync: false, __zone_symbol__xhrURL: "https://localhost:44385/api/auth", __zone_symbol__loadfalse: null, __zone_symbol__errorfalse: null, __zone_symbol__xhrListener: ƒ, …}
timeStamp: 80234.59999999614
total: 0
type: "error"
__proto__: ProgressEvent
headers: HttpHeaders
headers: Map(0) {}
lazyUpdate: null
normalizedNames: Map(0) {}
__proto__: Object
message: "Http failure response for (unknown url): 0 Unknown Error"
name: "HttpErrorResponse"
ok: false
status: 0
statusText: "Unknown Error"
url: null
__proto__: HttpResponseBase

Update 2:

Request with Postman:

Update 3: trying to get token several times showed that sometimes post request is successfull and sometimes not...