Hmm, perhaps this was an unpopular opinion, because the entire German military strategy depended on an extraordinarily fast decisive victory in France [1]. Of course, there were contingencies based on whether Russia was allied with France, or netural. Once the Schlieffen Plan fell apart, then the Germans knew it would be a long two-front war, but they didn't seem to suspect that in the first several weeks, from what I've read
Though I'll have to check out the book you mentioned, Pandora's Box.
I had difficulty hunting down exactly who thought it would be over quickly, although from what I remember from watching/reading about the war (for example I know it was talked about in at least one video of The Great War Youtube series), there were military and political leaders (non-German) that did seem to think they could end the war quickly.
"Experts and those in charge" and "public journalists and propaganda" are two distinct categories of people.