Why are my slow mo clips in Final Cut Pro going black?

While editing in Final Cut Pro (Version 11) on my Mac Studio M1 Max all of my videos recorded in 60fps or 24fps that I've slowed to 40% speed have turned black. In the storyline, it shows the correct thumbnails, but the inbetweens displayed are simply a pitch black screen.

When I switch to Optical Flow, it works, and for some clips, there is no virtual diference between optical flow and slo mo machine learning, and slow motion "good".

At some point it worked, for some clips, but for some reason it not longer working even in my MBA M1.

In screen appears "analyzing slo mo machine learning" but in the "tasks" window it isnt any activity (Activity Monitor also doesnt shows any activity in FC)

I reset preferences and tried a new Library in the MacStudio SSD drive