Hi, i have tested it on different providers (banan.cz, casablanca.cz, webzdarma.cz and still the same problem. After plugin activation i go to Menu - Widgets. All available widgets above the widget "ListCategoryPostsWidget" are movable. Your widget is not movable at all and all widgets below are movable, but the box has 100% width of the screen and you can not add them to Widgets.
I have setup sample at http://hospody5.mysteria.cz/wp2/wp-admin
admin/admin where you can see it. There is some older version but it behaves the same with latest versions of your plugin and wordpress too.
NOTE: Don´t try to update the plugin there - it would not work and delete the plugin..