You've done something wrong for sure. It's almost impossible to say what is wrong there.

First try to deactivate wpForo and activate it back, then delete all caches and check it.

If the issue is not fixed, go to Dashboard > Settings > Permalinks admin page, make sure the perma-structure is not the default ?p=1 one and save it. Then go to Dashboard > Forums > Settings > General Tab, click the button. And click the Visit Forum link in the Forum Base URL option section.