The snap worked, but only so so:
Now ffmpeg didn't find its component ffprobe, despite it being part of the snap.
God, I hated Linux in that moment!
So, away with the snap.
Read the ffmpeg documentation, found the list of its libraries, reinstalled them all and then, finally, everything worked.
But: Am I right or not?
This is not something a normal person would consider a viable solution.
3/3