OMG, I have found the solve.
Admins and developers don't make attention on this problem because they use EN version and don't see "Empty slug" error. But many users with another frontend language have this error. Just check "virtuemart Empty slug" on google.
Solve in file \administrator\components\com_virtuemart\models\product.php, it needs to comment lines 1725 - 1730.
But in needs to solve again after update. And don't delete products anytime, this is the reason of this problem appears.