Jellyfin detects wrong aspect ratio

[Efficient_Good_5784]

In my understanding Reencording is a loss of quality. Or is there a way to make it lossless?

Each time you encode with a lossy format, you lose some quality.

Re-encoding from an already re-encoded file would cause additional loss.  There are lossless modes for H264 and, I think, HEVC.  But the bit rates are insane.  If this is the original rip, as long as your encoding settings aren't stupid, the loss in quality would be minimal.  

Yeah, lossless for both H264 & HEVC is achieved with the CRF set to 0.

I would try re-encoding with sane values. You can play back the final encode(s) to see if you're happy with the outcome. This will probably be the easiest solution unless someone comes and shows how to edit the video's playback resolution without re-encoding.