OMA on Front End doesn't connect to backend

Environment:
FRONTEND: Front end server, Exchange 2003 Std. SP2, on Windows 2003
Std.
BACKEND: Backend Server, Exchange 2003 Ent. SP2 cluster, on Windows
2003 Ent, SP1 cluster
AD.DOMAIN.COM - Active Directory namespace
DOMAIN.COM - public namespace

Front end server was the single server previous to the cluster
implementation, and OMA worked fine in that configuration.

Front end server has forms based auth, and requires SSL, so the
alternate DAV virtual directory has been created and the registry
change applied (was done when the front end was the only server).
Front end server is also and RPC Proxy server (and always has been).
All other Exchange services (MAPI, RPC proxy, SMTP, OWA) work
flawlessly through the FE.

When I attempt to access http://FRONTEND/oma I get a login prompt and
then the following error:
Unable to connect to your mailbox on server BACKEND. Please try again
later. If the problem persists contact your administrator.


This error appears in the front end server's Application Log under the
title MSExchangeOMA:
Request from useruser@xxxxxxxxxxxxx resulted in the Microsoft(R)
Exchange back-end server IMAGOEXCH1 returning an HTTP error with status
code 409

Response:
Connection: close
Date: Thu, 01 Dec 2005 17:24:47 GMT
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
Content-Type: text/html
Content-Length: 140
MS-WebStorage: 6.5.7638

If I try to conenct to the FE via ActiveSync (with or without SSL), I
get Error HTTP_500.
I get the following the the App log of the FE server under the title
Server ActiveSync:

Unexpected Exchange mailbox Server error: Server:
[BACKEND.ad.domain.com] User: [user@xxxxxxxxxx] HTTP status code:
[409]. Verify that the Exchange mailbox Server is working correctly.

If I connect directly to the backend server either with the OMA virtual
directory with IE or ActiveSync, everything is kosher. This, of
course, defeats the purpose of having an FE.

Any help would be appreciated.

Thanks.

.

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