jQuery Ajax data server capacity problem with 503 errors

Question

I am working with WordPress, attempting to POST an Ajax request that contains a JSON string of approximately 5000 characters. It returns a 503 error, suggesting a capacity issue.

503 Service Unavailable The server is temporarily unable to service your request due to maintenance downtime or capacity problems.

I've reduced the JSON string to 2000 characters and it's working well without any errors. The post_max_size in the php.ini file is already set to 128M. I'm uncertain about where to investigate further.

var fields = JSON.stringify(obj); // more than 4000 characters

    $.ajax({
        url: 'admin-ajax.php',
        type: 'POST',
        data: {
            action: 'save_fields',
            fieldsData: fields
        },
        success: function(response){
            
        }
    })