OMA a system error has occurred

Issue with Outlook Mobile Access...(OWA works fine)

https://sub.domain.com/oma
prompted for username and password, then get:
A System error has occurred while processing your request. Please try again. If the problem persists, contact your administrator.

[EVENT VIEWER LOG ENTRY]
Event Viewer log entry:
An unknown error occurred while processing the current request:
Message: The remote server returned an error: (400) Bad Request.
Source: Microsoft.Exchange.OMA.ExchangeDataProvider
Stack trace:
   at Microsoft.Exchange.OMA.ExchangeDataProvider.OmaWebRequest.GetRequestStream()
   at Microsoft.Exchange.OMA.ExchangeDataProvider.ExchangeServices.GetSpecialFolders()
   at Microsoft.Exchange.OMA.ExchangeDataProvider.ExchangeServices..ctor(UserInfo user)

Message: Exception has been thrown by the target of an invocation.
Source: mscorlib
Stack trace:
   at System.Reflection.RuntimeConstructorInfo.InternalInvoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean isBinderDefault)
   at System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes)
   at System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes)
   at Microsoft.Exchange.OMA.UserInterface.Global.Session_Start(Object sender, EventArgs e)

Message: Exception of type Microsoft.Exchange.OMA.DataProviderInterface.ProviderException was thrown.
EventMessage:
UserMessage: A System error has occurred while processing your request. Please try again. If the problem persists, contact your administrator.
Source: Microsoft.Exchange.OMA.UserInterface
Stack trace:
   at Microsoft.Exchange.OMA.UserInterface.Global.Session_Start(Object sender, EventArgs e)
   at System.Web.SessionState.SessionStateModule.RaiseOnStart(EventArgs e)
   at System.Web.SessionState.SessionStateModule.CompleteAcquireState()
   at System.Web.SessionState.SessionStateModule.BeginAcquireState(Object source, EventArgs e, AsyncCallback cb, Object extraData)
   at System.Web.AsyncEventExecutionStep.System.Web.HttpApplication+IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)


For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
[/EVENT VIEWER LOG ENTRY]




System configuration is:
SBS 2003 Premium SP1
Exchange 2003 SP2
ISA 2004 SP3

I have tried several MS KB's to no avail:
http://support.microsoft.com/default.aspx?kbid=817379
http://support.microsoft.com/default.aspx?kbid=883380

For brevity, those KBs refer to running CEICW which should create/fix ExchangeVDir registry entries and recreating the default Exchange/OWA/OMA virtual directories.

Nothing is getting blocked in ISA.