Hello guys !
I just downloaded the latest version of Shotcut (20.04.12) and I am new to video editing 
I am trying to export a small mp4 video (1280x720), using the lossless H.264 preset with a 4K resolution (3840x2160). However, the resulting file turns out to be corrupted and cannot be played. I don’t know why, but when I decrease the quality from 100% (crf=0) to 99% (crf=1), the resulting file turns out to be OK.
Could you guys tell me what is wrong ? Thanks
Quality at 100% is interpreted as “use the lossless encoder” which is very different than the lossy encoder for 99% and lower. The file is not corrupt… the problem is that the media player you’re using doesn’t know how to read and play lossless files because they are less common. This is typical of most media players. If you imported that file back into Shotcut, you would see it play just fine.
On a separate note, 99% quality is probably too high for your needs. 68% is generally as good as the human eye can detect. Beyond that is overkill unless there is a specific reason. If you actually need lossless or intermediate level quality, consider DNxHR HQ or Ut Video. They have presets on the Export page.
What are you trying to play it with? VLC or MPC should handle something like that without issue.
Why are you scaling a 720p clip up to 4k? that’s a 9 fold increase in pixel count. If you are going to do something like that I’d adjust your interpolation setting to something better 
and as @Austin mentioned 100% with h264 isn’t something you’d ever distribute, 55% is “main profile” and indistinguishable from my 4k source without really focusing on it although I do make an FFV1 for archival purposes if I ever want to cut a clip for future projects.
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