last night when I was playing Descent, we had an argument about what happens when a hero makes an inturupt attack with a knockback weapon (mace of aver) on a blood ape in midleap. Would it knock the ape out of its leap or would knockback not take effect?

You can not interrupt a leap in mid leap per the newest FAQ. See Below:

Q: When making a Leap attack, is the leaping figure subject to Guard attacks, attacks produced by abilities such as Alertness, and Aura? If the figure is damaged by any of those effects, may it apply its Berserk ability to its attack roll (assuming it has Berserk)?
A: Yes on most counts. Leaping figures are immune to Aura, but otherwise subject to all the above noted effects. It may apply its Berserk ability if it takes damage during its leap. Note that if the figure is slain by any of these effects, then it may not make an attack roll at all. Guard may only be activated before or after a Leap attack is performed, never during.

Hope this helps

Brian

They had tried to impliment rules, but due to some claims by some of severly unclear issues they decided to change it so that Guard attacks have to be made before or after a Leap never during.