I broke tmux
Normally I like making things, but on occasion I've been known to break them.
Today it was tmux.
As it turns out, some updates change the protocol. When this happens, the updated application can't connect to sessions on the old protocol.
Luckily, there's an easy fix, at least in Linux:
/proc/$(pgrep tmux)/exe kill-server
Hope you didn't have anything important there! If you did, you can use:
/proc/$(pgrep tmux)/exe attach # Manually kill the required programs, cleanup, etc