You recently detached from your active tmux sessions using the [Ctrl]+b followed by d key combination, which is the standard method to safely leave a session running in the background. However, upon attempting to reattach to your sessions using tmux attach, you encountered an error indicating that no current sessions exist. Furthermore, when running tmux list-sessions, you received a message stating that no server is running on /private/tmp/tmux-501/default, despite the fact that this socket file exists, albeit with a size of 0. This can understandably be confusing, especially since htop shows active tmux processes that have been running for a significant amount of time, suggesting your sessions are still alive.

This issue typically arises when the tmux client is unable to communicate with the server through the expected Unix domain socket. On macOS, especially older versions like OS X 10.11.16, temporary directories (such as /private/tmp) can be cleared, rotated, or changed between sessions, leading to mismatches in the environment. Additionally, if the original tmux server was launched under a different user context, environment, or with a custom socket path (tmux -S), the default tmux client commands will be unable to detect or interact with the existing sessions.

To resolve this, you can begin by identifying the actual socket in use by the running tmux server. Running lsof -U | grep tmux in the terminal will list Unix sockets currently used by tmux, which may reveal an alternate path. Once identified, you can manually reattach to the session by specifying the correct socket with a command such as tmux -S /actual/path/to/socket attach. You can also confirm the running tmux server's process and environment using ps aux | grep tmux.

If no method allows reconnection and you determine the sessions are inaccessible, you may choose to terminate the tmux server using killall tmux, but be advised this will end all active sessions and should only be used as a last resort. To avoid similar issues in the future, consider launching tmux with a named socket using a known path, such as tmux -S /tmp/mytmuxsocket new -s mysession, to ensure session consistency across different terminal environments.