Thank you!

I will read that. always happy to learn more..

Depending on how in depth you want to go, here's a bit of a primer on Tg of dairy powders, that touches a bit on the phenomenon over time.  Little dated, but still decent info.

Solid and Liquid States of Lactose - Springer

It's likely a combination of denatured milk protein and amorphous lactose (having gone past it's glass transition temperature).  Keep your temperatures below 70C on your milk products and you shouldn't see this.  There is no way that i know of to process that hard material out once it has been formed.  Prevention is the key here.

yes. very. it took me hours to get rid of them.

some of the big particles cause the engine to stop when they stuck under the wheels. There are more smaller particles in the chocolate, most of them taste like burned milk powder and a few like caramel (from sugar). 

It is plausible to assume that the milk powder burned, however I would expect to see that from the first moment and not after a few hours when the particles are smaller and they need to stick to each other randomly to create clusters..

are the 'pieces' very hard and granular?

I assume you right and the secong option make sense.

The real question is why after a few hours and not in the beginning, moreover, it seized when grinding..

My guesses:

1) You've got water coming in somewhere

2) You denatured your milk protein when you baked it (if your temp really was 30C higher than 75C - then I'd lean in this direction)

Hello,

In contrast to previous two tries, the following had terribly failed:

I carmelized sugar separately. I mixed cacao liquer and some cacao butter with non-fat milk powder (as grainy stage or dry stage) and baked in the oven for 1.5 hours. I put the temperature on 75C but assume that it was 30C higher.

I grained everything (including the sugar) in my Santha 11 - it worked well for the first couple of hours but then seize and stick to the wheels (attached).

I tasted them and it feels like milk powder and not something else.

What had happened? and why it took couple of hours? the temperature of the chocolate was no over 50C.