CSS issues with gallery overwaping sidebar

Hello team,

I had this strange problem on my staging site.
https://staging.44sou.eu/gallery/gal1/
I’m using twentyfourteen theme and I have a menu and some widgets in the left sidebar but when I open a gallery it all is hidden.

Any hint what might cause this problem?
Thanks in advance.

https://www.ads-software.com/plugins/gallery-plugin/