This happens after opening a k2 item for editing in JCE. Whether or not the edit is done or not in the lightbox the lightbox is not closing when pressing close. In stead the previous window is showing up in the lightbox size and the same window outside is grayed out. Pressing the closing x in the right upper corner does not help. The way out is to press the back button in the browser several times.
This odd behavior only happens in front end edit.
J 2.5.4 - K2.5.6 - JCE 2.0.21

What is going on?

Probably javascript confilct, try changing the k2 javascript settings

Thank you for the advice. It solved my problem