Prior to SP4, no special right was required to impersonate a client.
Starting with SP4, the Impersonate a client after authentication user right is required for the account that will run the impersonation program.
To resolve this problem:
1. Use the Local Security Policy snap-in from the Administrative Tools folder on your Start menu.
2. Expand Local Policies.
3. Press User Rights Assignment
4. Double-click Impersonate a client after authentication in the right-hand pane.
5. Press Add on the Local Security Policy Setting dialog.
6. In the Select Users or Group dialog , select the user account that you want to add and press Add.
7. Press OK and OK.