Exchange OMA issue

I am having major issues with OMA, I have a windows mobile 5 that
synchronises well, however the sent items do not synchronise correctly.
and I get this error on the server

Unexpected Exchange mailbox Server error: Server:
[PHMSERVER.phantommusic.local] User: [dave.pattenden@**********] HTTP
status code: [500]. Verify that the Exchange mailbox Server is working
correctly.

I am unable to access the oma folder on the server, it does prompt with
a username and password box, however when I do enter the same username
and password for the Outlook Web Access I get this error ::: A System
error has occurred while processing your request. Please try again. If
the problem persists, contact your administrator

I have added the extra exchange-oma folder in IIS and also setup the
registry settings for that folder...
I am now stuck please help

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