Problem with plugin on the Widgets page

I have a problem – sidebar on a widget’s page stuck over the right corner of the available plugins, so i can’t add some plugins into sidebars. Everything was fine untill patch 1.2 or 3.5.1 version of WordPress (don’t know exactly). Can smbd help me with this prob ?

http://ww.wp.xz.cn/extend/plugins/custom-sidebars/