If I use the link to "forum" from NWN - EE within my games list, I only get to the general GoG forum.
To find this forum I had to use the Diamond Edition.

This is not uncommon. They lose the link, and don't update it. It's GOG after all. What do you expect.

I go to Browse All Games and search the game and go to the store page. There is a link on the store page for the forum.

Want to make a wager that this time next year the link still doesn't work? I could use some extra cash.