"Forum*" works for me (without quotation marks) but returns 10,347 search results - 690 pages of it.
But yes, just "forum" returns no results because "the search word is too common". I'm not actually sure there's much I can do about that, either. I probably don't have much control over the forum's search functions. It isn't broken, it just doesn't accept most words because they're "too common". It also won't do "return only posts that contain all
the words specified". I've wanted it to do that before, but it will return any post that has any
of the words specified, no matter what I do.