[Dovecot] Virtual mailbox help

[Joy]

Dear All,
              I am trying to configure Virtual mailbox but having
issue my squirrelmail is showing following error:-

ERROR: Connection dropped by IMAP server.
Query: LSUB "" "*"

I have also check Thunderbird but having same issue. Please check my
configuration and let me know any issue detected in it, Here is my
configuration:-

# 2.0.0: /usr/local/dovecot/etc/dovecot/dovecot.conf
# OS: Linux 2.6.26-2-686 i686 Debian 5.0.5
mail_location = maildir:~/Maildir
mail_plugins = quota acl  virtual

namespace {
  inbox = yes
  location =
  prefix =
  separator = .
  type = private
}
namespace {
  list = children
  location = maildir:/var/mail/public
  prefix = Public.
  separator = .
  subscriptions = no
  type = public
}
namespace {
  location = virtual:~/Maildir/.virtual:LAYOUT=maildir++
  prefix = virtual.
  separator = .
}

plugin {
  acl = vfile:/usr/local/dovecot/dovecot-acl/acl
}

After doing all configuration i created the following dir under .virtual dir.

/home/domain/user/Maildir/.virtual/.Unseen
/home/domain/user/Maildir/.virtual/.Unseen/
here created  dovecot-virtual file with following text ..

INBOX
  Unseen


Restarted and set proper permission on all created directories then
tried to login to client and getting the follwoing error in log file
..

Sep 09 13:02:47 imap-login: Info: Login: user=<piyush>, method=PLAIN,
rip=127.0.0.1, lip=127.0.0.1, mpid=3505, secured
Sep 09 13:02:48 imap(piyush): Panic: file strfuncs.c: line 72
(p_strndup): assertion failed: (max_chars != (size_t)-1)
Sep 09 13:02:48 imap(piyush): Error: Raw backtrace:
/usr/local/dovecot/lib/dovecot/libdovecot.so.0 [0x9fefd9e1] ->
/usr/local/dovecot/lib/dovecot/libdovecot.so.0 [0x9fefda5f] ->
/usr/local/dovecot/lib/dovecot/libdovecot.so.0(i_error+0) [0x9fefdd18]
-> /usr/local/dovecot/lib/dovecot/libdovecot.so.0 [0x9ff1a2df] ->
/usr/local/dovecot/lib/dovecot/libdovecot.so.0(t_strndup+0x30)
[0x9ff1a310] -> /usr/local/dovecot/lib/dovecot/lib01_acl_plugin.so
[0x9fd32747] ->
/usr/local/dovecot/lib/dovecot/libdovecot-storage.so.0(mailbox_list_iter_next+0x12)
[0x9ff559f2] -> dovecot/imap [0x8051deb] ->
dovecot/imap(cmd_list_full+0x424) [0x8052d34] ->
dovecot/imap(cmd_lsub+0x19) [0x8053129] -> dovecot/imap [0x805671c] ->
dovecot/imap [0x80567b9] -> dovecot/imap(client_handle_input+0x2d)
[0x805692d] -> dovecot/imap(client_input+0x5f) [0x805731f] ->
/usr/local/dovecot/lib/dovecot/libdovecot.so.0(io_loop_handler_run+0xf5)
[0x9ff0a275] ->
/usr/local/dovecot/lib/dovecot/libdovecot.so.0(io_loop_run+0x30)
[0x9ff09250] ->
/usr/local/dovecot/lib/dovecot/libdovecot.so.0(master_service_run+0x2a)
[0x9fef672a] -> dovecot/imap(main+0x2b5) [0x805ff95] ->
/lib/i686/cmov/libc.so.6(__libc_start_main+0xe5) [0x9fd66455] ->
dovecot/imap [0x804eac1]
Sep 09 13:02:48 master: Error: service(imap): child 3505 killed with
signal 6 (core dumps disabled)


Please let me know the issue ??