[Dovecot] server is not imap4 compatible

[marco ghidinelli]

hello,

i have a problem with my dovecot installation.

sometimes, the thunderbird clients on windows machines complains
about: "server is not imap4 compatible".

on the server, at the same time, i found this error:

Apr 20 16:21:17 harlock dovecot: imap-login: Disconnected: Shutting down: rip=192.168.0.194, lip=10.70.0.1, TLS handshake

the imap server is behind a dmz which sometimes drops connections.

the server ran without problem until i upgraded from 1.0.rc15 to
1.0.15.

after the upgrade i was forced to enhance the
proc.sys.fs.inotify.max_user_instances = 1024
because i had error messages about "Inotify instance limit for user
exceeded"

my question is: 
does the upgrade triggered a new way of communication between 
clients and server that exacerbates the problem of dropped
connections from the DMZ server?

p.s.
the dovecot configuration is:

$ sudo dovecot -n 
# 1.0.15: /etc/dovecot/dovecot.conf
log_timestamp: %Y-%m-%d %H:%M:%S 
protocols: imap imaps pop3 pop3s
ssl_cert_file: /etc/ssl/certs/harlock_digicert_2009.pem
ssl_key_file: /etc/ssl/private/harlock_digicert_2009.key
login_dir: /var/run/dovecot/login
login_executable(default): /usr/lib/dovecot/imap-login
login_executable(imap): /usr/lib/dovecot/imap-login
login_executable(pop3): /usr/lib/dovecot/pop3-login
login_max_processes_count: 256
mail_privileged_group: mail
mail_location: maildir:/var/mail/vhosts/turboden.net/%n/
mail_executable(default): /usr/lib/dovecot/imap
mail_executable(imap): /usr/lib/dovecot/imap
mail_executable(pop3): /usr/lib/dovecot/pop3
mail_plugin_dir(default): /usr/lib/dovecot/modules/imap
mail_plugin_dir(imap): /usr/lib/dovecot/modules/imap
mail_plugin_dir(pop3): /usr/lib/dovecot/modules/pop3
imap_client_workarounds(default): tb-extra-mailbox-sep
imap_client_workarounds(imap): tb-extra-mailbox-sep
imap_client_workarounds(pop3): outlook-idle
pop3_uidl_format(default): 
pop3_uidl_format(imap): 
pop3_uidl_format(pop3): %08Xu%08Xv
auth default:
  mechanisms: plain login
  default_realm: DOMAIN.LOCAL
  username_format: %Ln@%Ud
  passdb:
    driver: pam
  userdb:
    driver: static
    args: uid=5000 gid=5000
  socket:
    type: listen
    client:
      path: /var/spool/postfix/private/auth
      mode: 432
      user: postfix
      group: postfix