I tried to render a video with my Nvidia gpu. Sadly it ruins the video quality .
With the standard mp4 rendering profile (properties=x264-medium f=mp4 vcodec=libx264 acodec=aac g=120 crf=%quality ab=%audiobitrate+'k') the rendered video is fine. With the nvidia rendering profile (properties=x264-medium f=mp4 vcodec=h264_nvenc acodec=aac g=120 crf=%quality ab=%audiobitrate+'k') the rendered video is very bad. Rendered with the normal mp4 profile my test video was 700MB big, with the nvidia profile only 35MB.
My other rendering settings: video 15, audio 192, encoder speed max (to the right), 8 Threads (they are the same with mp4 and nvidia).

My system:
i7 7700K
16GB RAM
GTX 1080 (Nvidia 440.59 driver)
Kubuntu 18.04.4 (Linux 5.3.0-28-generic kernel)
Kdenlive 17.12.3

I tried some other nvidia profiles (properties=x264-medium f=mp4 vcodec=nvenc_h264 acodec=aac g=120 crf=%quality ab=%audiobitrate+'k' preset=fast AND f=mp4 movflags=+faststart vcodec=h264_nvenc progressive=1 g=15 bf=2 crf=%quality acodec=aac ab=%audiobitrate+'k'), same problem.

What can I do? Do I make something wrong?

I assume you enabled MOVIT. Please disable it and try again. MOVIT is not stable and leads to strange rendering issues. We hope we can implement GPU accelerated rendering during this year.

use snap packs
Kdenlive + ffmpeg(of the permissions)
Everything works
I don't use movit