Thanks alot for this tip!
I had the same problem and solved it. The message I was getting was "No wemux server to attach to on 'wemux'".
1. the first problem I seemed to have was when changing the default name in the wemux config. I re-commented that line so the only line I had effectively was for the allowed host. Then my host could run a server.
2. the second problem seems to be that the socket directory has the wrong group permissions. According to the readme:
Use wemux start to start a wemux server, chmod /tmp/wemux-wemux to 1777 so that other users may connect to it, and attach to it. If a wemux server already exists, it will attach to it instead."
Mine had the group permission set to 0. If I chmod that directory as instructed, and call "mux SESSIONNAME" as usual, it works.