side window like sample crud application

Hey

For my thesis i am using vaadin and studied the sample crud application of vaadin. i applied that feature which allows you to click on a grid and open a side window to show additional informations, change data or to delete the object.
my problem is that if i connect the application with two different broswers only the one of the browser can open the window.
looks like:
Browser1: Login->Button click->side window opens
Browser2: Login->Button click->side window opens
now the weird thing is after i opened the app with a second browser i can't open the side window on browser1
Browser1: Button click->side window doesn't open
Browser2: Button click->side window open (still working)

I really don't see why it doesn't work and i hope someone can help me with this issue