Updated: I found and used ZELLIJ_SOCKET_DIR in the meantime.

Recently I tried to use zellij on a shared remote Linux server (which I don't have root permissions).
zellij failed to start (which no output since it cleared the terminal buffer). I straced that down to:

1268:mkdir("/run/user/1000/zellij/0.41.2", 0777) = -1 ENOENT (No such file or directory)
1269:mkdir("/run/user/1000/zellij", 0777)    = -1 ENOENT (No such file or directory)
1270:mkdir("/run/user/1000", 0777)           = -1 EACCES (Permission denied)

$XDG_RUNTIME_DIR is /run/user/1000. However, that path doesn't exist on the server,
and even if it does, it isn't writable by normal users. Curious, I typed tmux ls and found that
tmux uses /tmp/tmux-1000/default as server path.

Could zellij fallback to somewhere in /tmp too?
Alternatively, I could kindly ask admin to support $XDG_RUNTIME_DIR.
But I don't know the infrastructure admin too well to know how long it would take.