Confirmed. There is an issue here. I started a project with the nightly build from two days ago and in that project I took one small clip and slowed it down in Properties to 0.25 and I reused that slowed down clip 3 times. I was able to export the project in that same session but when I closed it and opened that mlt file with the nightly build from 2 days ago I got the exact message that @dailylolrecap posted up when I pressed the Export button. It wasn’t until I deleted all 3 instances of that clip that I slowed down to 0.25 that I was able to export. The only way I was able to export the mlt file with the slowed down clip after closing it was opening it with v20.11.28.
So there is some big problem happening here but I don’t know what the exact trigger for it is because when I tried to reproduce it by starting a new project, making sure to slow down a clip and opening the project after saving it I could export it. So something along with slowing down the clip in Properties is causing the error message in export.