MAC:Entourage 2004 - cannot login on Exchange 2007 server

[Krabke]

We have an iMac Intel computer with OS X 10.5.6. With Mail (POP and SMTP) we
can send and receive e-mails, using the mailserver.

If we try to logon onto the same Mailserver with Entourage, using the
Exchange account we cannot, alltough the address list does work via the LDAP
server.
The returned error is:
The server for account "xxxx" returned the error "Logon failure: unknown
user name or bad password." Your username/password or security settings may
be incorrect.

Wefound some solutions on internet but none of them helps.

Any suggestions or known bugs?

The support company for our server (Exchange 2007) cannot help us, they seem
to be scared of our MAC ;-)