Okay, something must have happened since from a certain point on, my defence stat got drastically lowered. From around 130-140 it decreased to 98 and it wont increase above that no matter what. And there is more: even if I start putting some more points into dexterity nothing changes. Every 2 point in dexterity should increase defence by 1 point but it doesnt happen.

Any idea whats going on? Is there an enemy in act 3 that could have caused this? I know there are enemies that can cause permanent stat decrease, but nothing in the game log said anything (I have been keeping an eye on this). If its indeed an enemy that caused this happening, then it must have completely broken the game since dexterity doesnt increase defence anymore...

I'm not sure if there is an enemy in Act III that permanently decreases stats, but even if there was one he shouldn't affect the possibility to increase them again. What happens when you strip all your equipment? Does (Dexterity + Speed)/2 = Defence? If it does, the math should be all right and the rest of your Defence should come from your equipment - its Armor Class value and its +X to all attributes bonus. Does that add up?