setgid failed: Operation not permitted

[Ed Swierk]

I'm running a CVS pserver as a non-root user (cvs) on host cvs.  I have 
created a mapping for each user in the CVSROOT/passwd file:

  user1::cvs
  user2::cvs
  ...

A few users are getting a "setgid failed: Operation not permitted" error, 
but only when accessing the repository via SSH port forwarding (ssh -L 
2401:cvs:2401, with CVSROOT=:pserver:address@hidden:/cvs).  Everything 
works when accessing the repository directly (with 
CVSROOT=:pserver:address@hidden:/cvs).

The server version is 1.11.5.  Every file in the repository is owned by 
group cvs (which is the primary group of user cvs).  Should the files be 
owned by user cvs as well?

Any ideas what the problem might be, or how to go about tracking it down?

--Ed