Got Out of sync error on the Drag components demo

Dear Vaadin community,

I am testing the Drag components demo as a Portlet on Liferay framework.

When I try to drag and drop one component I get :

Out of Sync error and, on the web server log I have:

Warning: Ignoring variable change for non-existent component, VAR_PID=DD

Do you have any cue on what should I change on the demo code to solve this problem?

Thanks in advance,

Eva.