Retrofit Android - Expected a string but was BEGIN_OBJECT (When parsing request BODY)

I have a problem with Retrofit & Android.

I have API method "api/account/login". This method has Content-Type application/json and what about the body:

"Request body has to contain LoginData object serialized to JSON format and properly encrypted" (from api doc) So, sample body:

"cUdu2LkEChP...Lw7R9m4="

Also, I have second API Method: "api/account/data", which have the same body and content-type.

What is my problem? I am using Retrofit to connect with API.

interface APIService {
@POST("api/account/login")
void postLogin(@Body String encryptedCredentials, Callback<String> callback);
@POST("/api/account/data")
void postLoginData(@Body String encryptedCredentials, Callback<String> response);
}

for first method everything works fine, API returns what should return, but, for the second method Retrofit returns error:

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected a string but was BEGIN_OBJECT at line 1 column 2 path $

Actually now I have no idea what is going on, both methods take the same body, but only one is failing...

handling api-calls:

@Override
// THIS WORKS FINE!
public APIRequest postLoginRequest(final String encryptedCredentials) {
    return new APIRequest() {
        @Override
        protected void fireRequest() {
            APIServiece.postLogin(encryptedCredentials, new Callback<String>() {
                @Override
                public void success(String s, Response response) {
                    logicEventBus.post(new LoginEvent(null, s));
                }

                @Override
                public void failure(RetrofitError error) {
                    logicEventBus.post(new LoginEvent(error, null));
                }
            });
        }
    };
}

@Override
// THIS DOESNT WORK FINE :(
public APIRequest postLoginDataRequest(final String encryptedCredentials, final AccountData.LoggedBy loggedBy) {
    return new APIRequest() {
        @Override
        protected void fireRequest() {
            Callback<String> callback = new Callback<String>() {
                @Override
                public void success(String s, Response response) {
                    logicEventBus.post(new LoginDataEvent(null, s));
                }

                @Override
                public void failure(RetrofitError error) {
                    logicEventBus.post(new LoginDataEvent(error, null));
                }
            };

            switch (loggedBy) {
                case INTERNAL:
                    APIService.postLoginData(encryptedCredentials, callback);
                    break;
                case FACEBOOK:
                    APIService.postLoginData(encryptedCredentials, callback);
                    break;
                case GOOGLEPLUS:
                    APIService.postLoginData(encryptedCredentials, callback);
                    break;
            }
        }
    };
}

I have checked in the debugger, strings are fine. API also works fine (checked in Postman). Debugger never get into "success" (in not-working method).

Thanks for any kind of help!