Nevermind, I figured it out.
I had an old “Forums” directory on the server.
It got lost in the shuffle because it was my primary domain on a shared hosting account, and I didn’t see it amongst all of my other domain folders.
I wanted to add here that I tried creating another page, containing a shortcode, for the forum index:
http://www.forgreenpeas.com/forum-index
And that works fine. However, If I try to click on the “Forums” link in the breadcrumbs, it takes me to the other forums page that doesn’t work:
http://www.forgreenpeas.com/forums
Still not sure what’s causing this.