tmux won't run as non root user?
Solution 1:
A more general way to solve a problem of this nature:
strace -f -e trace=file tmux ..restofyourcommand...
Look for permission denied / no such file or directory errors right before it exits - might find a general reason behind why you're getting this error (that is, see what directory it was trying to create, then make it manually and observe the error, it might be that it's trying to make /home/x/some/dir without /home/x/some existing or something).