steps to reproduce using the attached example:

1) click the button "Show notification!" -> notification error is shown -> OK
2) close the modal (not the notification)
3) now try to close the notification
-> it won't close and further clicks will be blocked by a stackup iframe having a higher z-index
1800 (notification div) vs 1801 (stackup iframe)

by default this problem only happens in IE, but in this example "zk.useStackup=true" is set to also reproduce it in FF and Chrome

WORKAROUND:
disable stackup by setting the JS variable

zk.useStackup=false

e.g. by adding this script to the page

<?script content="zk.useStackup=false"?>