The first time I run Avidemux after the PC boots, it crashes. If I run it again, it works. I am using version 2.7.1 appimage on Ubuntu 18.04. Here is the output in a pastebin since it didn't fit: https://pastebin.com/MNXiHeWa Unfortunately crash.py is empty.
Is the package libssl1.0.0 installed?
Apart from that, do you use the version 2.7.1v2 of appImage? The first one didn't work properly on Ubuntu 18.04: http://fixounet.free.fr/avidemux/news.html