Tmux [exited] on startup

Removing

set-option -g default-command "reattach-to-user-namespace -l zsh"

from the configuration fixed the problem for me.

I ran into this problem, and the reason was that the shell path was wrong in my .tmux.conf

set -g default-shell /usr/local/bin/fish

It couldn't find the shell, so it crashed. You should probably check whether your default shell's path is correct.