AppImage loading error: can't start qtractor

let me tell you a story :)

an AppImage's is built and provided for just a very first/turn-key trials of the software and quite mostly for so called mainstream Linux distros.

in fact, the AppImage's being supplied from sf.net (which are my own stuff:)) are all based on the oldest currently still ubuntu LTS (16.04) and that might, just often might clash with some system libraries on your latest-and-greatest-top-bleeding-edge 2020 Linux system :)

but again, for too many reasons to list here, some of those bundled libraries might not be fully ABI-compatible with some more recent baseline system libraries--it just takes one single flipped bit to fail--namely the infamous moving targets of libav* vs. (official) ffmpeg and their (in)direct dependencies, as it sure seems to be your dang case.

Anyway, you did succeed to build your own qtractor binary from source then, go with it! It's the best and greatest you may get on your particular system, no BS, no worries!

AppImage is not a perfect, nor is any other bundle/container/whatever system...

on a GNU/Linux world, if it doesn't work, use the source (Luke! ;))

cheers