WebException: The request failed with HTTP status 401: Unauthorize

Replica servers are not synchronizing to master server.

Here is the error generated by the replica server:
WebException: The request failed with HTTP status 401: Unauthorized.
at
System.Web.Services.Protocols.SoapHttpClientProtoc ol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
at System.Web.Services.Protocols.SoapHttpClientProtoc ol.Invoke(String
methodName, Object[] parameters)
at
Microsoft.UpdateServices.ServerSyncWebServices.Ser verSync.ServerSyncProxy.GetAuthConfig()
at
Microsoft.UpdateServices.ServerSync.ServerSyncLib. InternetGetServerAuthConfig(ServerSyncProxy proxy, WebServiceCommunicationHelper webServiceHelper)
at
Microsoft.UpdateServices.ServerSync.ServerSyncLib. Authenticate(AuthorizationManager
authorizationManager, Boolean checkExpiration, ServerSyncProxy proxy, Cookie
cookie, WebServiceCommunicationHelper webServiceHelper)
at
Microsoft.UpdateServices.ServerSync.CatalogSyncAge ntCore.InternalGetConfig()
at
Microsoft.UpdateServices.ServerSync.CatalogSyncAge ntCore.CatalogSyncThreadProcessReal(Boolean allowRedirect)

Any help would be appreciated.