[Dovecot] Spurious " Renaming not supported across conflicting directory permissions"

[tlhackque]

Dovecot 2.1.10
Client = Thunderbird.  Local disks.  mbox format.

Attempted to rename a folder, failed with:

   CANNOT Renaming not supported across conflicting directory permissions

I don't understand why this should happen.
   o I was renaming within a directory - the attempt was to rename 
"Domain Names &- SSL Certificates" to "Domain Names".  No directory was 
specified
   o The reason the directory has g=s is for dovecot - so that the 
'mail' group (12) will stick to any files created.  I was getting more 
understandable permission failures before I added it.  Since 's' doesn't 
apply to files, I don't see why this should be viewed as a conflict.
   o If I chmod g-s the directory, the command succeeds.  But then I'm 
back to file ownership problems...

drwxrws--- 4 4503 12 4096 Sep 22 06:17 F/N/
-rw-rw---- 1 4503 12 81530 Sep 22 06:17 F/N/Domain Names &- SSL Certificates

Sequence from TB's trace (crlfs for clarity):

1140[11ebcf00]: 121e8c00:imap.example.com:A:SendData:
    66 rename "F/N/Domain Names &- SSL Certificates" "F/N/Domain Names"
1140[11ebcf00]: ReadNextLine [stream=12135a28 nb=80 needmore=0]
1140[11ebcf00]: 121e8c00:imap.example.com:A:CreateNewLineFromSocket:
   66 NO [CANNOT] Renaming not supported across conflicting directory 
permissions 9592[c7d6840]: ReadNextLine [stream=ab687e8 nb=22 needmore=0]

Config:

# 2.1.10: /usr/local/etc/dovecot/dovecot.conf
# OS: Linux 2.6.22.14-72.fc6 i686 Fedora Core release 6 (Zod)
first_valid_gid = 4000
first_valid_uid = 4000
hostname = smtp.example.com
login_greeting = Dovecot ready.  Unauthorized access is prohibited.
mail_location = mbox:~/mail:INBOX=/var/mail/%n
mail_privileged_group = mail
namespace inbox {
   inbox = yes
   location =
   mailbox Drafts {
     special_use = \Drafts
   }
   mailbox Junk {
     special_use = \Junk
   }
   mailbox Sent {
     special_use = \Sent
   }
   mailbox "Sent Messages" {
     special_use = \Sent
   }
   mailbox Trash {
     special_use = \Trash
   }
   prefix =
   separator = /
}
passdb {
   driver = pam
}
service imap-login {
   inet_listener imap {
     address = imap.v4.example.com imap.v6.example.com
     port = 143
   }
   inet_listener imaps {
     address = imap.v4.example.com imap.v6.example.com
     port = 993
     ssl = yes
   }
}
service pop3-login {
   inet_listener pop3 {
     address = pop.v4.example.com pop.v6.example.com
     port = 110
   }
   inet_listener pop3s {
     address = pop.v4.example.com pop.v6.example.com
     port = 995
     ssl = yes
   }
}
ssl_ca = </etc/pki/tls/certs/example_com_ca-cert-crl.pem
ssl_cert = </etc/pki/tls/certs/servers-example-com.pem
ssl_key = </etc/pki/tls/private/servers-example-com.pmk
userdb {
   driver = passwd
}
protocol imap {
   imap_client_workarounds = tb-extra-mailbox-sep tb-lsub-flags
}
protocol pop3 {
   pop3_client_workarounds = outlook-no-nuls oe-ns-eoh
   pop3_uidl_format = %08Xu%08Xv
}

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