Premiere Pro CC 2017 - black rectangles after rendering

Hello everyone, I am facing the issue: After rendering a video, random black rectangles are appearing in the video. They are in the same spot, at the same time after different rendering settings. I want to get rid of them, but they are not in the preview. The video is in 60fps (some sequences are made slower, but rectangles are not appearing there), 1080p. I tried to render it in 30fps, but no change. What is that and how I can solve this problem? Thank you.