I have seen a similar problem on a Sun system, don't know if it's
relevant here.
>From the SunOs 4.1 manual for /etc/chsh :
The new login
shell must be one of the approved shells listed in
/etc/shells unless you are the super-user. If /etc/shells
does not exist, the only shells that may be specified are
/bin/sh and /bin/csh.