problem with the CLOSE button of the plugin Pop Up!

Hello,

I have a problem with the CLOSE button of the plugin Pop Up!

I fixed display:none because I want to remove it but seems that the class is not stable and changes after a timeframe not defined.

Please look at picture attached linked below.

https://www.dropbox.com/s/619lp0vudetltj8/close_class.jpg

Thanks

Simone