question about plugin subscribe2

I’ve installed the mailinglist plugin subscribe2, from the following site:
https://www.skippy.net/blog/category/wordpress/plugins/subscribe2/
It works, and it even shows the header, footer and sidebar of my blog, BUT the sidebar starts at the end of the mailinglistpage. I’ve looked into the php-files, checked the <div> and </div> codes, but can’t find what is wrong.

Does anyone know a solution perhaps?
To see it in action as it is right now, see:
https://www.popindekop.nl/home/subscribe.php

Thanks for helping!