wp admin - Failed to load resource: the server responded with a status of 500 (Internal Server Error)

I'm getting this 500 error in the browser:

HTTP500: SERVER ERROR - The server encountered an unexpected condition that prevented it from fulfilling the request. (XHR)POST - .php

Below is my code. This gets executed when I press the submit button. When the submit is clicked, this alert: alert('!!!Submission Error!!!'); pops up followed by the above error on the console. I couldn't figure out what's happening. I have the same copy of the site in my staging environment, where it doesn't show any error.

$.ajax({
        url: jquery_main_data.ajaxurl,
        data : {
            action : jquery_main_data.action_form,
            fields : fields,
            nonce: jquery_main_data.nonce_form
        },
        type : 'post',
        dataType: "json",
        beforeSend: function() {
            jQuery(".button.submit").text("Loading...");
        },
        success: function(response) {
            console.log(response);

            if(response.is_valid == true) {
                    window.location.href = '/thank-you/';
            } else {
                alert('error....');
            }
        },
        always: function() {
            jQuery(".button.submit").text("Submit");
        },
        error: function(errorThrown){
            alert('!!!Submission Error!!!');
            jQuery(".button.submit").text("Submit");
            console.log(errorThrown);
        }

    });