Unable to connect to the Synchronization Service.

Error:

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Synchronization Service Manager
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Unable to connect to the Synchronization Service.

Some possible reasons are:
1) The service is not started.
2) Your account is not a member of a required security group.

See the Synchronization Service documentation for details.

Reason:
The Forefront synchronization service user does not have access rights in the required permission groups.

Solution:
1. Open the "Server Manager"
2. Go to the Groups Folder
3. Select the "FIMSyncBrowse" group  and double click on it.
4. Add the respective user and Click "OK" button.
5. Do the same steps to add the same user for "FIMSyncJoiners", "FIMSyncOperators", "FIMSyncPasswordSet" groups.
6. Logoff your server and Login again.
7. Now you can see that if you click on miiclient.exe file, it will be displayed without showing error.

For More information, see the below screenshot:

That's it...