if i stabilize a clip (this create a second video with .mlt extension)
and i drag the stabilized video on timeline, when i apply the "Speed" effect, the video just disappear from the timeline (visually) but it will played without any speed change.

Something similar happens to me. I apply the effect of "Speed" on reverse archive .mlt and I move the file through the timeline and the effect is left erroneous. When I try to apply "Stabilize" the program is closed. Version 17.12.0d but I have tried several previous versions and there is the same problem.

yep.. seems that multiple effects/transitions have a lot of issues...

I can confirm this issue is still present in 20.08.1.

This still happens on Kdenlive 21.04 how can a bug report be filled?

here: https://invent.kde.org/multimedia/kdenlive/-/issues