Wrong page shown on post request

With plugin enabled:
If you make a form request on the page, then the main one will be displayed. If you go to this URL without a post-request, the desired page will be displayed
With plugin disabled:
Everything works without problems. Exactly the page that is needed is displayed.

What could be the problem? There is no way to disable the plugin on specific pages, they are important for optimization.

Thanks for the help!

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