When using the default winbind separator '\', wbinfo --set-auth-user fails
. I have these two lines in smb.conf:

   #winbind separator = \
   winbind use default domain = no

So, with that line commented out, the default separator setting is selected.

  test:~# testparm -v -s | grep "winbind separator"
          winbind separator = \

Unfortunately:

  test:~# wbinfo --set-auth-user="DOMAIN\Administrator"
  could not obtain winbind separator!
  Password: 
  test:~# echo $?
  0
  test:~# wbinfo -t
  checking the trust secret via RPC calls failed
  error code was  (0x0)
  Could not check secret

To get it to work, I can do either:

   #winbind separator = \
   winbind use default domain = yes

   test:~# wbinfo --set-auth-user="Administrator"

Or:

   winbind separator = /
   winbind use default domain = no

   test:~# wbinfo --set-auth-user="DOMAIN/Administrator"

There is a related problem in that I can't explicitely set the winbind separator
 to '\' in the config file (but I'll file that as a separate bug).