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SharePoint Calendar gives that error below when using SharePoint Exchange Calendar overlay.

The HTTP request is unauthorized with client authentication scheme 'Ntlm'. The authentication header received from the server was 'Negotiate,Basic realm=""'

We couldn't find any solution for this problem. Does anyone have any idea about the problem?

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Are you using OWA URL when you're connecting the calendar in SharePoint? – Jussi Palo Jan 23 '12 at 14:19
Yes, i am writing additional information, OWA URL: Exchange Web Service URL: – Serkan Jan 23 '12 at 15:13

Please make sure to add the Root certificate in you SP Farm. Here are the steps to do it:

1) Go to "Central Administration" and click on "Security"

2) Under "General Security" click on "Manage trust"

3) Create a name (e.g. TestExchange)

4) Root certificate for the trust relationship:

5) Click OK

For more information on how to add root certificate see this post

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We already did this its not the solution. – Serkan Jan 23 '12 at 15:17
Could you please check if Exchange OWA can be set up to use Basic or NTLM authentication. I would guess that SP is expecting NTLM, and Exchange is set to use Basic. – Falak Mahmood Jan 23 '12 at 16:18

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