bbPress.org

Very strange! I was setting up my new forum (newest WP, newest bbpress) and the subforum pages are looking just fine. Also the topic pages looked good – until recently. Now, the topic page is only displaying the core content (starting with the breadcrumb-path of the forum topic), but no design at all!

When I look at the sourcecode of a topic page, it starts like this:

Where is all the beginning of the page, with the usual html elements for the page design?

I activated another theme and it has the same problem.
Do I have to reinstall bbpress?

Unfortunately, I cannot post a link here, the forum is only visible for logged-in users.

Thanks for any help!
Nils