Last night, I actived cms on my live site. I am not sure that is bug or not but I want you know this.

If one forum category url starts with number like one ;
9. grade exam results
forum/9-grade-exam-results/

it is directed to content area like this;

forum/content/any-contents-thread-with-id-nine-9/

To fix
Romove the space on 9.grade
As you figured it out, forum category url changed, so I tried to Custom Redirects but has no effect