jmcunx maybe something cleared /tmp taking the tmux socket with it? Do you have some special script to handle /tmp in your ~/.profile and ~/.login?
Anyway, from tmux(1):
-L socket-name
tmux stores the server socket in a directory under
TMUX_TMPDIR or /tmp if it is unset. The default socket is
named default. This option allows a different socket name
to be specified, allowing several independent tmux servers
to be run. Unlike -S a full path is not necessary: the
sockets are all created in a directory tmux-UID under the
directory given by TMUX_TMPDIR or in /tmp. The tmux-UID
directory is created by tmux and must not be world read-
able, writable or executable.
If the socket is accidentally removed, the SIGUSR1 signal
may be sent to the tmux server process to recreate it (note
that this will fail if any parent directories are missing).
So, I'd try to recreate the socket first:
$ for i in $(pgrep tmux)
> do kill -USR1 $i
> done
