Yes, we too run phpBBS3 and WordPress on the same server.
Both are good, but different.
We have no difficulty with phpBB3 from outside or LAN, but as many posts attest, there appears to be an issue with WordPress.
We noticed that phpBB3 navigates by relative addressing from its front page, so as long as the root document can be found, the rest works fine.
WordPress, in their infinite wisdom, seem to use absolute addressing, so that the web address -appropriate for outside access- defines the front page, and all other pages. This obviously breaks when WordPress is accessed, not by http://www.something.com/wordpress/ from outside, but on the LAN it would appear something like 192.168.1.xxx/wordpress/ . So that explains why you can get to the WordPress front page but can't navigate from it on the LAN.
How to fix? Some fiddling with local name resolution with /etc/hosts and similar as described above has been reported as successful, but we can't make it work reliably with all of WordPress, and without causing problems elsewhere.
Maybe a WordPress guru here knows how to persuade WP into relative addressing, but I don't know that it is possible.
This issue has been posted so many times, asking for a solution and closed without a practical answer, it seems worth getting to the bottom of it. WordPress is magnificent, but IMHO this is an irritation that needs a fix..