>so the hatred survived very intact through the entire Soviet era.

where is your evidence that this hatred predates the Soviet era?

I live in ex-Yugoslavia where myths(perceived from outside) of "ancient ethnic hatred" are pervasive, while in reality they cannot be traced earlier then the 20th century.

The peoples here yearned for a united country of the south slavs (Jugoslavia) for centuries, then started killing each other within a few decades of forming that country for quite prosaic reasons mostly related to personal liberties and dividing state incomes and expenses

I doesn't take ancient ethnic hatred to start a war, scarce water or land resource will do it just as well, and it will create enough "new ethnic hatred" to last a century