imap_open authenticating with cyrus imap

[hep]

Dear all,

I have made a web mail with php.
It has worked fine until we update the cyrus imap to a new version.
Imap_open can not anymore connect to the server.
Error
Oct 16 10:33:59 www cyrus/imapd[7107]: badlogin: www.arelec.ch[144.85.15.72] CRAM-MD5 [SASL(-13): user not found: no secret in database]

Here are the version of software that we use:

• - debian 3.0
  - cyrus21-imapd 2.1.9-1
  - apache 1.3.26-0woody1
  - php4 4:4.1.2-5
  - cyrus21-imapd 2.1.9-1
  - php4 4:4.1.2-5

And this is the call to imap_open():

 //Faster to use IP adress than hostname
  $MailServer = "{144.85.15.72:143}$Folder";
  $mbox = imap_open($MailServer,$SESSION_USER_NAME, $SESSION_USER_PASS);

For me, it looks that imap_open try to connect with secure way (MD5), but cyrus is confugurated tu connect with plain. If somebody can tell me hwo to connect using imap_open with plain ...