So,

I solved it, but if anyone would care to enlighten me about the technical reasons of what happened, here it is:
The block I built as a map entity was marked as DETAIL on its property tab.
After I unchecked it DETAIL flag the Malfunction error W1508 was gone and now I am able to load two instances of the same map entity.
But why?
What does the DETAIL feature implies to an entity?