Only Firefox TypeError: "NetworkError when attempting to fetch resource."

I have a page which loads content dynamically, and I make several calls to backends API's all these calls have the same configuration, like this:

function get_data_1(){
  var data = {
    type: "home"    
  };
  
  fetch('../backend/get/slider.php',  {
      method: 'POST',      
      credentials: 'same-origin',
      body: JSON.stringify(data),
      headers:{
        'Content-Type': 'application/json'
      }
    })
    .then(function(response) {      
      return response.json();
    })
    .then(function(response) {            
      if( response.success ){
       //here proceses de code
      } else {               
       //here proceses de code if not success response
      }
    })
    .catch( function(error){      
      console.error(error);
      //Here is where the error catch and show the error: NetworkError when attempting to fetch resource
  });   

When the page load, call many functions like this:

get_data_1(); get_data_2(); ...

All the responses of the backend API have the same output format (JSON) and have the same headers, I'm using an only server (localhost), but only some calls get the error and others work fine, the problem is only with Firefox with chrome, opera, and edge I no have any error.