Help on jQuery form (going to error stage)

Hi, my code is below.

For some reason, upon clicking submit, the code reaches this bit:

error: function(XMLHttpRequest, textStatus, errorThrown) {			alert('Sorry! An error occurred while sending. Please try again later.');
			$submit.attr('disabled', '');

Code (markup):

I am unable to work out why - any help is greatly appreciated as this is driving me crazy!

Please feel free to let me know if you require any further info.

Full code:

lse.preregister = function($form) {
	var $submit = $form.find('input[type="submit"]');
	var regType = $form.find('select').val();
	if (regType == 0) {
		alert('Please choose a registration type!');
		return;
	}
	var pattern = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+[a-zA-Z]{2,4}$/;
	if (!pattern.test($form.find('input[name="email"]').val())) {
		alert('Please input a valid e-mail address!');
		return;
	}
	if (!$form.find('input[name="firstname"]').val()) {
		alert('Please type your first name!');
		return;
	}
	if (!$form.find('input[name="lastname"]').val()) {
		alert('Please type your last name!');
		return;
	}
	if (!$form.find('input[name="organisation"]').val()) {
		alert('Please type your university / organisation!');
		return;
	}
	if (!$form.find('input[name="position"]').val()) {
		alert('Please type your degree level / position!');
		return;
	}
	if (regType % 2 == 0) {
		if (!$form.find('input[name="course"]').val()) {
			alert('Please type your course name!');
			return;
		}
		if (!$form.find('input[name="year"]').val()) {
			alert('Please type your year in university!');
			return;
		}
	}
	/*$submit.attr('enabled', 'enabled');*/
	$.ajax({
		url: $form.attr('action'),
		type: $form.attr('method'),
		data: $form.serialize(),
		dataType: 'json',
		success: function(data) {
			if (data.succeed) {
				window.location.href = './Payment';
			}
			else {
				alert('Sorry! An error occurred while sending. Please verify the form again.');
				$submit.attr('disabled', '');
			}
		},
		error: function(XMLHttpRequest, textStatus, errorThrown) {
			alert('Sorry! An error occurred while sending. Please try again later.');
			$submit.attr('disabled', '');
		}
	});
};

Code (markup):