I am skeptical as well. One could argue that debt (internal and external) is inversely correllated with state strength, and that war is a way of reinforcing state strength. One might argue that this is part of the grounds for the war of 1812 (the segment of the population that backed the war was motivated in part by their loathing for British creditors).
Ultimately there are books written about why nations go to war, and I believe the only consensus is that there is no single reason why nations go to war. The set of reasons varies by country and by decade. (Any answer for the period of the Congress of Vienna is probably not applicable to the post WWII period, nor can answers from the current era be compared with answers from the bipolar cold war regime.
I also expect that it would be tough to prove, since I sincerely doubt that the agressor would admit "We're going to war because we're deadbeats!".
However you could probably assemble a dataset of wars and graph the trade deficit between the initiating parties. (I think that allies would cloud the issue); the graph would be an interesting resolution to the question.