Event ID - 648

Event Id648
SourceMicrosoft-Windows-ADFS
DescriptionThe Federation Service encountered an error while loading the trust policy. A partner is configured with more than one group-to-UPN transformations that use the same group. If this error occurs during startup of the Federation Service, the Federation Service will be not be able to start, and all requests to the Federation Service will fail until the configuration is corrected. If this error occurs while the Federation Service is running, the Federation Service will continue to use the last trust policy that was loaded successfully. User Action This error should occur only if the trust policy file has been modified without use of the AD FS administrative tools. No two group-to-UPN transformations on one partner may use the same group.
Event Information According to Microsoft :

Cause

This event is logged when the Federation Service encountered an error while loading the trust policy.

Resolution :

Specify only one group-to-UPN mapping for a group

This error occurs only if the trust policy (trustpolicy.xml) file has been modified without the use of the Active Directory Federation Services snap-in and the modification causes another relying parameter to fall outside the scope of acceptable values. In this case, the manual change that was made to the trustpolicy.xml file specifies that two or more group-to-UPN mappings have been assigned to use the same group.

In the trustpolicy.xml file, search for GroupToUpnClaimTransformation XML elements, and make sure that two or more elements do not have the same value for the Group element.

Verify :

Verify that you can access the Active Directory Federation Services (AD FS)-enabled application from a client browser and that the resource can be accessed.
Reference LinksEvent ID 648 from Source Microsoft-Windows-ADFS

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