"Experts and those in charge" and "public journalists and propaganda" are two distinct categories of people.
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.
While I thought this was about COVID-19 before clicking, I assumed it would use WWI as an example of how experts and those in charge could get predictions of how long something would take really, really wrong, and was kind of disappointed to not see any mention of that in there.
https://www.historynet.com/over-by-christmas.htm