[Dovecot] Virtual folder root cannot be opened, virtual folders ok

[Tom Talpey]

I am running dovecot 1.2.16 and not able to figure out an error
that happens whenever I select the root of a virtual mailbox tree.

At the moment I have a simple maildir store, accessed via IMAP, and
have successfully added a virtual fs namespace with two subdirectories.
I have created dovecot-virtual entries in the subdirs and they are
working fine; they show the expected messages, etc.

My problem is with the top-level virtual directory. If I happen
to select it in Thunderbird (Eudora, actually), I receive a popup
	"The current command did not succeed.  The mail server
	 responded:Mailbox doesn't exist: Mobile."
It also fails in the same way during search, which interrupts it and
makes other client operations fail.

I have tried placing a dovecot-virtual rule in the directory, leaving
the directory empty, moving it in or out of the maildir tree, tweaking
namespace and INDEX/LAYOUT options all with the same result - the
subdirs are fine, the virtual root fails.

I must be missing something obvious. Clue, please? Thanks!

Relevant configuration:
[~] # dovecot -n
# 1.2.16: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.33.2 i686  ext4
...
mail_location: maildir:/var/MailRoot/domains/%d/%n/Maildir
mail_executable: /usr/libexec/dovecot/imap
mail_plugins: fts fts_squat virtual
namespace:
   type: private
   separator: /
   inbox: yes
   list: yes
   subscriptions: yes
namespace:
   type: private
   separator: /
   prefix: Mobile/
   location: virtual:%h/Mobile:INDEX=MEMORY
   list: yes
   subscriptions: yes
...

[~] # ls -lsaR /var/MailRoot/domains/<d>/<u>/Maildir/Mobile
Mobile:
    4 drwx------    4 xmail    xmail        4096 Nov 30 16:06 ./
   20 drwx------  243 xmail    xmail       20480 Nov 30 20:55 ../
    4 -rw-------    1 xmail    xmail          19 Nov 26 17:18 
.virtual-subscriptions
    4 drwx------    2 xmail    xmail        4096 Nov 29 10:54 Mail1/
    4 drwx------    2 xmail    xmail        4096 Nov 29 10:54 Mail2/
    4 -rw-------    1 xmail    xmail          14 Nov 30 16:06 
dovecot-virtual

Mobile/Mail1:
    4 drwx------    2 xmail    xmail        4096 Nov 29 10:54 ./
    4 drwx------    4 xmail    xmail        4096 Nov 30 16:06 ../
    4 -rw-------    1 xmail    xmail          27 Nov 26 17:16 
dovecot-virtual

Mobile/Mail2:
    4 drwx------    2 xmail    xmail        4096 Nov 29 10:54 ./
    4 drwx------    4 xmail    xmail        4096 Nov 30 16:06 ../
    4 -rw-------    1 xmail    xmail          22 Nov 26 17:16 
dovecot-virtual
[~] #