I already posted the issue we're having when opening topics here  https://wpforo.com/community/how-to-and-troubleshooting-2/sub-category-links-not-working/#post-6591  

I just realized that if I remove the last slash from the permalink custom structure the topics open just fine "/%postname%/" but the forums won't show any topics (known issue that is fixed by adding this slash)

Topics don't open the first time (404 error) but after second refresh they appear, but without the slash they open right away. 

This sounds like a cache issue but I removed WP Super Cache plugin, tried different browsers with 5 QA people at the same time and that happens in no matter what browser, we have 6000+ members and we can't find what the issue is.

Thanks.