I am too new to SF, so I am not allowed to post a comment. Sadly, I need to post this in the form of an answer...
Try running:
strace -o TMUX-DEBUG.txt tmux
strace output will get dumped to the file TMUX-DEBUG.txt.
If you haven't used it before, strace is an extremely useful program that tracks all system calls an executable makes. Chances are moderately high that you will see the specific function call that causes this error, which can lead you to the device it's trying to call, or even more useful clues.
Basically, strace allows you to iniate extremely powerful "detective" work when trying to track these kinds of things down.
There's also 'ltrace', which tracks user-level calls. It too can be useful. Let me know if strace shows anything that jumps out at you. Good luck.