bbPress.org

Hi,
This is WP 4.8.1 with bbP 2.6rc3 (was hoping this corrected it, was running 2.5.13 all the time). THE CORE from ThemeFurse is running an Lifter LMS is doing the Site-Protection.
That’s the entrance https://youngstars.leben-fuehren.de/wp-login.php and “demouser” is username and password.
I can see the forums (/foren/) and the topics (e.g. “Mastermind”) /foren/forum/masterminds/. The Topics headline have a Link to the topic itself, e.G.: /foren/topic/test-topic/
BUT when I click on the headline, the topic doesn’t open. It goes back to the topics list /foren/forum/masterminds/.
I can create answers in the backend. But the frontend doesn’t show any.

• I did deactivate all Plugins -> no change
• I went back to twenteseventeen -> no change
• I fiddled around with the permalinks ->no change
• I repaired bbPress and BuddyPress several times -> no change

Funnily enough: [bbp-single-topic id=2279] works. I don’t get it and start to panic here, I guess. Any idea welcome!
Thanks
OLAF