Search phrase length can't be more than 100 chars. Also minimum length of any word is 3 in "Any words" type of search and at least one word must be more than 2 chars in any other type of search, except for the case, when you're searching within number of days and the "days" value doesn't exceed default "days" value for this community - the one, which you see here in the "days" input area right after clicking on the "Search" link.
