It looks like they like... that. Don't know why it's returning a 404 when accessing the forums without the index.php part. All I could think of is either the SEO is messed up or there's an actual index.php folder.
Possibility.
Log into sites WordPress admin
Go to Settings -> Permalinks.
Note the setting.
Set to 'Default' and save changes.
Visit site. See if domain name on links correct.
Go back to Settings -> Permalinks and reset to original. Save settings.
Visit site to check. Click around and check links.