WordPress admin-ajax.php gives a 500/400 status when called with custom code

I created a small script to handle a dropdown select that retrieves data from a custom MySQL table. The script uses admin-ajax.php, and it works perfectly on my local MAMP. However when I uploaded the files on Bluehost server I have a 500 status for the request if I am logged in as administrator. If I ma not logged in, I have a 400 bad request. Moreover the page appears with the administration top bar, even if I am not logged in and it appears to other users as well. I have no idea of what happened

Here's my code

<script>
jQuery(document).ready(function($) {
    // Event listener for changes in the first dropdown (#region_filter)
    $('#region_filter').on('change', function() {
        // Fetch selected region values
        var selectedRegions = $(this).val();

        // Make an AJAX request to retrieve corresponding areas for selected regions
        $.ajax({
            url: "domain.org/wp-admin/admin-ajax.php",          
            type: 'POST',
            data: {
                action:'eAtlasFiltersRegions',
                regions: selectedRegions
            },
            success: function(response) {
                // Parse the JSON response
                var areas = JSON.parse(response);

                // Clear and update the third dropdown (#isra_filter) options
                var israDropdown = $('#isra_filter');
                israDropdown.empty();
                israDropdown.append($('<option value="">Search area names</option>'));
                israDropdown.append($('<option value="all">SELECT ALL</option>'));

                // Add retrieved area names to the dropdown
                areas.forEach(function(area) {
                    israDropdown.append($('<option value="' + area + '">' + area + '</option>'));
                });

                // Refresh the dropdown to apply changes
                israDropdown.dropdown('refresh');
            }
        });

    });
 
});
</script> 

and on the functions.php I wrote the following:

add_action('wp_ajax_eAtlasFiltersRegions', 'eAtlasFiltersRegions');

function eAtlasFiltersRegions(){
    global $wpdb;
    
    if (isset($_POST['regions'])) {
      // Get the selected regions from the AJAX request
      $selectedRegions = $_POST['regions'];
      console.log($selectedRegions);

      // Construct the SQL query to retrieve areas based on selected regions
      $query = "SELECT DISTINCT area_name FROM isra_map_area_names WHERE region_code IN (SELECT region_code FROM isra_map_regions WHERE region IN (";
      $regionValues = array_map(function($region) {
          // Sanitize and escape each selected region
          return "'" . addslashes($region) . "'";
      }, $selectedRegions);
      $query .= implode(",", $regionValues) . ")) ORDER BY `area_name` ASC";
      // Execute the query
      $query_results = $wpdb->get_results($query);
  
      // Extract area names from the query results
      $areaNames = array();
      foreach ($query_results as $result) {
          $areaNames[] = stripslashes($result->area_name);
      }
  
      // Return the area names as a JSON response
      echo json_encode($areaNames);
  } else {
      // Return an empty JSON response if no regions are selected
      if (isset($_POST['regions']) & $_POST['regions'] =='') {
        $query_area_name = $wpdb->get_results("SELECT * FROM isra_map_area_names ORDER BY `area_name` ASC", ARRAY_A);
         // Extract area names from the query results
      $areaNames = array();
      foreach ($query_results as $result) {
          $areaNames[] = stripslashes($result->area_name);
      }
  
      // Return the area names as a JSON response
      echo json_encode($areaNames);
      }
  }
  wp_die();  

}

The wordpress environment is the same on local and on server, as well as the plugins. Has anyone found the same issue? What is the mistake in my code?

From what I can see, your code as two errors:

1. console.log is in your PHP file, that will cause issues.
2. in the else part of your if-else statement, you are looping through $query_results when your results were assigned to $query_area_name.

Give it a try, and see if that fixes your 500 error. If your 500 error goes away and you still have issues, check out this resource:

https://stackoverflow.com/questions/48025825/wordpress-admin-ajax-php-400-bad-request?rq=2