I was experiencing crashes related to my .tmux.conf, this is what I did to figure out:
Run tmux to get client, server and output log files.
tmux -vvvvv
cat tmux-out-*.log
This will give you a hint of what is going on, if for example your config file wasn't parsed properly and the program has crashed before you were able read something.
Client and server logs could be accessed this way:
cat tmux-client-*.log
cat tmux-server-*.log