Alert messages from Notification Center hang after using close button

After opening my MacBook in the morning, Notification Center displays the alerts from my Calender (I realy like this new feature!).

Problem

When I click the Close Button on an Alert Message the Alert Message does not close but stops responding and stays displayed.

The only way to get rid of it is to logout and login to OSX.

Does anyone have the same problem?

I am looking for a solution.

All suggestions are welcome!

Thanks,

Chris