You are right!
Actually, zone multipliers seem to be not related to window multipliers, it is DB which has a default option of lumping similar windows into one window for the same surface and applies window multipliers automatically. I think that's why the simulation crashed!
As a first test I just set window multipliers to 1 in IDFEditor and it worked! I will export the geometry again without that DB default lumping option and report back the result here.
Hisham