Approving topics in moderation is not working
I just installed it, trying to approve a topic but when I click on approve, it just goes to a blank page like you can see in the screenshot.
This is not a proper way to find the problem's cause.
Follow the standard WP procedure for debug. Part of if was posted above by Robert. Enable WP Debug.log also.
I'm not sure what you mean by "not proper" way. This is obviously a bug in the plugin, it seems like the developer simply copy pasted his code above it:
$uids = array_diff($uids, (array) $wpforo->current_userid);
There are already several reports about this issue. And the admin's response was:
This is a site specific problem end comes from other plugins and theme in most cases. To find the reason please deactivate all plugins and temporary change the theme if it's possible.
So it would take some times for the admin to act on this simple bug.