Your back certainly shouldn't be rounding with less than 200kg. To look at a guy like Konstantinovs and try to adopt aspects of his form as a beginner is doing it very wrong. There are 100s of adaptations that have occurred over many years of training that make lifting like that possible. If your upper back is rounding at this stage, it's fairly likely that your lumbar is moving too.
As a beginner, you need to be focussing on having your back straight. Even when you start lifting over 2xbodyweight your form should be tending towards promoting a neutral spine.
Direct advice: be stricter with your form, and stop the set when you feel it slipping. It might be worth strengthening your upper back, but it should take care of itself.