I just ran into the same exception as this thread.
I am not using Turnkey and had set-up 2 users using useradd name -g mc
.
This does not add those users to the group like a supplementary group. I suspect the default mc user in the other thread is using mc as its login group too.
In mounts.py
, getgrnam(group) returns an empty array of members in this situation
Doing usermod name -G mc
adds the user to the group list in /etc/group
and creating a server works.