The Fourth Crusade: a Plot?

Was there a "plot" to divert the Fourth Crusade to Constantinople, and if so who was responsible?
"We will never know, and science has truly something better to do than to discuss indefinitely an insoluble problem" -P. Mitrofanov
This famous comment on the fourth crusade and the question of a "plot" was made well over a century ago, and very little new evidence has emerged since. So why return to this topic?
Most modern historians are of the view that the diversion of the Fourth Crusade to Constantinople was not the result of a plot. Queller, for example, writes "By reflection upon a detailed examination of the Fourth Crusade... we can perhaps expand our understanding of mankind caught up in the great current of events" , and Riley-Smith, Mayer and Pernoud are all of more or less the same opinion. Indeed it cannot be proved, now or probably ever, that the diversion of the crusade was the result of a "plot"; but the mere lack of proof should not lead to a conclusion that there was no plot. It is not the duty of historians to treat subjects as "innocent until proven guilty" and there is no justification in rash, unproven conclusions such as that quoted above. …