[WutHirsch]

Not completely.
But the mythemeshop support helped me a lot.

The correct code is:

.bbpress .sidebar.c-4-12 {
  display: none;
}
.bbpress .article {
  width: 100%;
}

Also i was able to change the breadcrumb / the forum base title into another word.
Simply add this code into: bbpress.php:

// bbPress, change Forum title in bbpress 
 
function isa_custom_forum_title() {
 
     return 'Enter your title';
}
 
add_filter('bbp_get_forum_archive_title', 'isa_custom_forum_title');

[WutHirsch]

O.K. Guys, i find the problem by my own.
Typing Error, the right code is:

#bbpress-forums, .article, div#content_box.sidebar_layout {
	width: 100%;
}

But now my Start Page got a full width too, again.
Hmm, ok so i have to try to fix it again.
It would be easier if i would find the Page id from the forum root.

But i dont know, where can i find it.
Normaly i can see the page id in the urls.
Its not possible here.