I work with LRT 4.6 under windows 10. Yesterday I received my private licence key and installed it. I generated a lot of videos with the new features I couldn't used before. I generate with

- MP4
- Quality high
- LRT Plus Bewegungsunschärfe
- Schärfen

with 720p: with this resolution I can watch my video
with 1080p: with this resolution I see always a black screen

I started my computer and made the generation again, it doesn't help. The exported photos are not black, of course, only the video.

The other sequences I can watch the generated videos with 720p and 1080p.

It might be your video player. Some players can only deal well with videos in 16:9.
Please try using the MPC-Player: https://mpc-hc.org/
Another option would be to activate "Force 16:9 Output" when rendering the video. But MPC is always the best choice for playing.

Yes, with MPC-Player it plays right. But why does the other videos function with the windows-player? Maybe they are a little bit shorter!?

I assume it's because you did not render in 16:9.