I hear from doctors, contrary things about high protein diet affecting kidneys. what is the truth? is it harmful or not and when / who?

Depends on health of: It is dependent on the health of your kidneys. If your kidneys are normal, then high protein dies has no effect on them. But if you have impaired kidney function(with a high serum creatinine level(>2mg), then proteins are potentially harmful and low protein diet is useful and proven helpful. So high protiens are toxic to damaged kidneys.