When I build a new page, in this case, one titled "Forum Rules and Instructions" or alternatively: "Forum Guide" WordPress automatically inserts a URL slug: example: http://strokesite.com/forum-guide (see screenshot attached). 

A page with this slug that begins with "forum…" Will display a 404 – page not found title in the browser tab."

I have deleted the page several times and rebuilt it from scratch. Deleted my browser cookies and server cache, and the 404 appears every time. I even deactivated WPforo and restarted to no avail.

To solve it, I rebuilt the page and gave it a name that begins with something other than "Forum." It seems beginning a slug URL with "Forum" was the source of the problem. Now it works. Thought you would like to know.