Plugin does not work for “Specific Posts”

Hi,

I just wanted to create a new snippet for a blog post of mine.
When I choose the option “Specific posts” for “Site Display” the Post list is empty, so I can not select my chosen blog post.
I tested the function on two websites, with two different themes and webprovider.
Now I helped myself with setting a blog post category which works (somehow with double tracking code).

How can I select single posts instead of blog categories?

Thanks very much in advance 🙂

The page I need help with: [log in to see the link]