Error -17900 (authentication error) when trying to login to Exchange from Entourage 2004

Whenever I try to add an Exchange account to Entourage 2004, the error
-17900 appears and tells me that the username/password is wrong. The
same credentials however work fine in either Apple Mail 10.5, on an
iPhone or in Entourage 2008. We have some Hosted Exchange 2007
accounts with our ISP and they haven't experienced this.

It doesn't really matter if the account credentials are either right
or wrong, it simply doesn't work and always generates the same error.
I looked all over the net for this -17900 error but can't find it
anywhere. My provider also doesn't have a clue. Any of you have seen
this? It must be related to Entourage 2004.