Fusion issue from 2 movies

Hello :wave: (and 1st sorry for my poor english)
I came here because I have exhausted all my resources! For several months I have been trying to overcome a bug (for which I have not found any information) when I use MkvToolNix it is only to put 2 films and the muxes to have the video of one and the sound on the other.

About 2 months ago I could only do 1 movie every 2/3 days (the first succeeded without problem and if I want to do a second the progress bar goes to 100% in half a seconds and nothing is done)

I recently uninstalled everything including permissions and registry keys in order to start from new. Unfortunately nothing has to change except that now I can’t even do 1 :angry: (same problem, the progress bar reached 100% in less than 1 second without doing any processing… :weary:

Is there anyone please who can enlighten me?