Private forum not included in search

A common phpbb 3.0.3 installation with several forums,
one of them configured as "private", i.e. only the administrator group can see it and edit it: actually, they have "full access" on it. A search, aparently, never includes the contents from this private forum, even if the user making the search belongs to the administrator group.
And the "advanced search" page does not include it in the list of the allowed forums to be included in the search.

The user can search all the messages of another user who belongs to the Administrator group; the excerpts of the posts in the private forum appear correctly in the result page; but if when he clicks "Search these results" (using a word ostensibly present) it always displays "No posts were found because the word ... is not contained in any post."