High UID user can not login

[Stamper, Steve]

I have added a new user which requires a specific UID.
  useradd -c "Example Agent Account" -g eaa -d /eaa -u 6000 examp03

Trying to login using Putty I enter the ID and password then get:
  PuTTY Fatal Error - Server sent disconnect message: "Command terminated on
signal 25."

Using usermod I knock the UID down to a low order number and I can log in
correctly.
  usermod -u 511 examp03

This tells me there is probably a MAXIMUM UID so I looked in /etc/login.defs
and found:
  UID_MAX         60000
which is well above my 6000 number.  

Any ideas where I can look next???

Thanks
Steve