[Python] Flask-Login problems intermittently with ajax

I'm using Ajax to login, using Flask-Login extension. Here's my server side code:

@app.route('/login', methods=["POST"])
def login():
if current_user.is_authenticated:
redirect_url = url_for('index')
return jsonify(loggedIn=True, redirectUrl=redirect_url)

username = request.form.get('username', '').strip()
password = request.form.get('password', '').strip()
user = User.query.filter_by(username=username).first()
if user and util.encrypt_password(password, user.salt) == user.password:
logged_in_user = CurrentUser(user)
login_user(logged_in_user)
redirect_url = url_for('index')
return jsonify(loggedIn=True, redirectUrl=redirect_url)
else:
return jsonify(loggedIn=False, error='Invalid Email/Password')


and my client side code:

(function(){
$login_form = $('#login_form');

//add validation to form
if( $login_form.length ){

$login_form.parsley()
$login_form.submit(function(e) {
var url = $(this).attr('action');
var data = $(this).serialize();

tryLogin(url, data);
return false;
});
}

function tryLogin(url, data){
var $submitBtn = $('#login_form__submit');

//notify user that we are working
$submitBtn.addClass('btn--loading');

$.ajax({
type: 'POST',
url: url,
data: data,
dataType: 'json',
success: function (data) {
if (data.loggedIn) {
mixpanel.track('login_success');
window.location.href = data.redirectUrl || '/';
}
}
});
}
});


Not sure what's wrong, I have started seeing this problem lately, and it fails more than 50% of times. In case of failure, it'll just redirect to the new page but no session info exist. All the content is hosted on the same domain.

EDIT: More info: even when the login doesn't work intermittently, the backend does pass the login and frontend receives loggedIn=True and redirectUrl. Seems like issue is with session/cookie being received by the client but not sure why the intermittent issue.

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