I didn't make any changes. I think the issue may be that /forums may be internal Drupal path. Did you try disabling the Drupal forum module in core if you're not using it? That should force a 404 redirect but if you have your phbb3 installation below your Drupal root, Apache should find it. Also, check your Error reporting for 404 redirects. Do you have it set to go to the home page?