HTML shown in visual editor

Hi! I really love bbp style pack!

One problem: some HTML shows up in posts, like here: https://despelletjesvrienden.nl/forums/topic/wingspan/

I tried this so far:
1) Every code I could find of the forums and google
2) Different editors (HTML, Visual or both)

Unfortunately I can’t fix it. Could you help me?

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