“ My smb.conf file is correctly configured. I have specified idmap uid = 12000 and idmap gid = 3000-3500, and winbind is running. When I do the following it all works fine. ”

root# 

wbinfo -u

MIDEARTH\maryo
MIDEARTH\jackb
MIDEARTH\ameds
...
MIDEARTH\root

root# 

wbinfo -g

MIDEARTH\Domain Users
MIDEARTH\Domain Admins
MIDEARTH\Domain Guests
...
MIDEARTH\Accounts

root# 

getent passwd

root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/bin/bash
...
maryo:x:15000:15003:Mary Orville:/home/MIDEARTH/maryo:/bin/false

“ But this command fails: ”

root# 

chown maryo a_file

chown: 'maryo': invalid user

“This is driving me nuts! What can be wrong?”

Your system is likely running nscd , the name service caching daemon. Shut it down, do not restart it! You will find your problem resolved.