Last image not loading

Hello all,

I have a quastion. One of my websites has a problem with the gallery. The links is: http://www.dekkergroep.nl/producten-en-techniek/delfstoffen/industriezand-en-ophoogzand/

When you click on the image below the page, and go to the last image than you can see the problem. The slider keeps loading. This is a problem of all the slides on the whole website. But when i am in the back-end there are only 4 images. Afther image 4, the slider keeps loading but there is nothing to load.

Who can help me fix this problem?

Greats,
Dennis

https://ww.wp.xz.cn/plugins/nextgen-gallery/