standard murder configuration problem

[Maurizio Lo Bosco]

Hi All,
I'm going crazy configuring a standard murder architecture with 3 different 
servers
 1 frontend (mx1)
 1 mupdate master (mupdate)
 1 backend (ariel)
the OS is debian etch.
the imap/pop3 and lmtp connections are not working from the frontend to the 
backend


------- MX1 mail.log
Jul 15 15:23:27 mx1 cyrus/lmtpunix[8197]: couldn't authenticate to backend 
server: no mechanism available

Jul 15 15:26:05 mx1 cyrus/imap[8236]: couldn't authenticate to backend server: 
no mechanism available
-------
I can connect directly to the backend with a client and I have succesfull 
result with imtest and lmtptest from the frontend to the backend.

root at mx1:/etc# imtest -a helpdesk -u helpdesk ariel
S: * OK ariel Cyrus IMAP4 v2.2.13-Debian-2.2.13-10 server ready
C: C01 CAPABILITY
S: * CAPABILITY IMAP4 IMAP4rev1 ACL QUOTA LITERAL+ MAILBOX-REFERRALS NAMESPACE 
UIDPLUS ID NO_ATOMIC_RENAME UNSELECT CHILDREN MULTIAPPEND BINARY SORT 
THREAD=ORDEREDSUBJECT THREAD=REFERENCES ANNOTATEMORE IDLE 
MUPDATE=mupdate://mupdate/
S: C01 OK Completed
Please enter your password:
C: L01 LOGIN helpdesk {8}
S: + go ahead
C: <omitted>
S: L01 OK User logged in
Authenticated.
Security strength factor: 0
C: Q01 LOGOUT
Connection closed.

root at mx1:/etc# lmtptest -a murder -u murder ariel
S: 220 ariel LMTP Cyrus v2.2.13-Debian-2.2.13-10 ready
C: LHLO example.com
S: 250-ariel
S: 250-8BITMIME
S: 250-ENHANCEDSTATUSCODES
S: 250-PIPELINING
S: 250-SIZE
S: 250-AUTH PLAIN
S: 250 IGNOREQUOTA
Please enter your password:
C: AUTH PLAIN bXVyZGVyAG11cmRlcgBzM3JnMTA=
S: 235 Authenticated!
Authenticated.
Security strength factor: 0
C: QUIT
Connection closed.

What's wrong with my config?

mx1 cyrus.conf
-----------
START {
        recover         cmd="/usr/sbin/ctl_cyrusdb -r"

        delprune        cmd="/usr/sbin/cyr_expire -E 3"
        tlsprune        cmd="/usr/sbin/tls_prune"
}
SERVICES {
        sieve           cmd="timsieved" listen="localhost:sieve" prefork=0 
maxchild=100
        notify          cmd="notifyd" listen="/var/run/cyrus/socket/notify" 
proto="udp" prefork=1
        mupdate       cmd="mupdate" listen=3905 prefork=1
        imap            cmd="proxyd" listen="imap" prefork=0 maxchild=100
        pop3            cmd="pop3proxyd" listen="pop3" prefork=0 maxchild=50
        lmtpunix                cmd="lmtpproxyd" 
listen="/var/run/cyrus/socket/lmtp" prefork=1 maxchild=20
}
EVENTS {
        checkpoint      cmd="/usr/sbin/ctl_cyrusdb -c" period=30
        delprune        cmd="/usr/sbin/cyr_expire -E 3" at=0401
        tlsprune        cmd="/usr/sbin/tls_prune" at=0401

}
-------------
mx1 imapd.conf
-------------
configdirectory: /var/lib/cyrus
defaultpartition: default
partition-default: /var/spool/cyrus/mail
altnamespace: no
unixhierarchysep: no
lmtp_downcase_rcpt: yes
admins: cyrus murder
lmtp_admins: murder
mupdate_server: mupdate
mupdate_port: 3905
mupdate_authname: murder
mupdate_username: murder
mupdate_password: xxxx
mupdate_retry_delay: 10
proxy_authname: murder
ariel_password: xxxx
allowanonymouslogin: no
popminpoll: 1
autocreatequota: 0
umask: 077
sieveusehomedir: false
sievedir: /var/spool/sieve
hashimapspool: true
allowplaintext: yes
sasl_pwcheck_method: saslauthd
sasl_mech_list: PLAIN
sasl_minimum_layer: 0
sasl_auto_transition: no
tls_ca_path: /etc/ssl/certs
tls_session_timeout: 1440
tls_cipher_list: TLSv1+HIGH:!aNULL:@STRENGTH
lmtpsocket: /var/run/cyrus/socket/lmtp
idlemethod: poll
idlesocket: /var/run/cyrus/socket/idle
notifysocket: /var/run/cyrus/socket/notify
syslog_prefix: cyrus
------------------
Hope you can point me in the right way.
Regards
  Maurizio