Vaadin calendar - invalid json response

Hello, i tried to play with vaadin calendar a little bit and im facing this problem. I have my calendar i weekly view and when i try to move between weeks ( using arrows in corners ) i get "Communication problem Take note of any unsaved data, and click here or press ESC to continue.Invalid JSON response from server: "
My code looks like this:

protected void init(VaadinRequest vaadinRequest)
root = new VerticalLayout();
Calendar cal = new Calendar("Prominence calendar");

i’ve even tried to implement handlers for this but i was still getting same error.

EDIT: Actually i get same error when i for instance add event and move him by drag and drop.
Im using chrome 59.0.3071.115 but it is same on edge/explorer.
I’ve even tried this code
and i get same error. Can it be something with my app server ( i get it on tomcat as well as on glassfish) ?