It happened to me when I did an upgrade from 4.0.0 version to the 4.2.0 that all the pages got redirected to the activity.php files without displaying all the threads available in the forum. In order to get this fixed you can do the following to solve it!
Go to your admincp -> Navigation Manager. Under the tab "Forum" Click the options menu and select "Set default".
And with that you got this solved!