Empty 200 response from API's while using OAuth2

Hi, 
I am trying to make API calls with O Auth 2 Authentication but facing some issues, previously I used Authentication based on Cookie. So I followed the steps to create an O Auth 2 connection, create a client id, client secret with O Auth 2 Flow as ‘Resource Owner Password Credentials. The Bearer token is getting generated when I hit this access token URL “/identity/connect/token”. The issue is whenever I make an API call with the generated token I get a 200 response with No data (image attached).

First I got a response something like this :
<script>
    window.open("/Main","_top");
</script>
So I passed the Accept and Content-Type in the header and then started getting an empty response. 
(Note: A bulk import is happening from Acumatica using APIs, so is this because of API usage limits? I am not sure about this)
I'm new to this system and wonder if I'm missing something. 

Thanks