Exchange ActiveSync returned an HTTP 500 response.

I'm having trouble getting an iPhone to sync up with Exchange 2003 SP2.

using https://www.testexchangeconnectivity.com 

returns the following error:
 Testing Exchange ActiveSync  
  Exchange ActiveSync test Failed
   Test Steps
   Attempting to resolve the host name mail.xxxxx.com in DNS.
  Host successfully resolved
   Additional Details
  IP(s) returned: x.x.x.x  
 
 Testing TCP Port 443 on host mail.xxxxx.com to ensure it is listening and open.
  The port was opened successfully.
 Testing SSL Certificate for validity.
  The certificate passed all validation requirements.
   Test Steps
   Validating certificate name
  Successfully validated the certificate name
   Additional Details
  Found hostname mail.xxxxx.com in Certificate Subject Common name  
 
 Testing certificate date to ensure validity
  Date Validation passed. The certificate is not expired.
   Additional Details
  Certificate is valid: NotBefore = x/x/2009 10:03:27 PM, NotAfter = x/x/2010 8:37:47 PM"  
 
 
 
 Testing Http Authentication Methods for URL https://mail.xxxx.com/Microsoft-Server-Activesync/ 
  Http Authentication Methods are correct
   Additional Details
  Found all expected authentication methods and no disallowed methods. Methods Found: Basic  
 
 Attempting an ActiveSync session with server
  Errors were encountered while testing the ActiveSync session
   Test Steps
   Attempting to send OPTIONS command to server
  OPTIONS response was successfully received and is valid
   Additional Details
  Headers received: Pragma: no-cache
Public: OPTIONS, POST
Allow: OPTIONS, POST
MS-Server-ActiveSync: 6.5.7638.1
MS-ASProtocolVersions: 1.0,2.0,2.1,2.5
MS-ASProtocolCommands: Sync,SendMail,SmartForward,SmartReply,GetAttachment,GetHierarchy,CreateCollection,DeleteCollection,MoveCollection,FolderSync,FolderCreate,FolderDelete,FolderUpdate,MoveItems,GetItemEstimate,MeetingResponse,ResolveRecipients,ValidateCert,Provision,Search,Notify,Ping
Content-Length: 0
Date: Fri, 04 Dec 2009 00:00:09 GMT
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET

 
 
 Attempting FolderSync command on ActiveSync session
  FolderSync command test failed
   Tell me more about this issue and how to resolve it
   Additional Details
  Exchange ActiveSync returned an HTTP 500 response.  
 
 
I have gone through alanhardistys excellent article https://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/A_1798-Exchange-2003-Activesync-Connection-Problems-FAQ.html

This part in particular has me wondering...
" Remove SSL settings from the Exchange IIS virtual directory
" Run iisreset
" Test activesync without SSL selected - hopefully this should work or give the OK result

Even without SSL selected on the Exchange Virtual Directory - the connectivity test fails

" If okay - right-click on the Exchange Virtual Directory and select all Tasks> Save Configuration to a file. Name the file Exchange and save to the desktop

Any help would be greatly appreciated.