Fotoxx appimage cannot launch ffplay process

I am trying to get the Fotoxx appimage to run on Ubuntu 17.10. I had to add some files from /usr/share/mime to get the pixbuf loaders to work. Apparently these files have changed format between Ubuntu 16.04 and 17.10. Another problem is still open: the program ffplay works OK on Ubuntu 17.10, but when Fotoxx (appimage version) launches ffplay using a system() call, ffplay outputs “unknown error” and quits. This works OK on earlier Ubuntu releases. What is going on?