I am getting error "**Sending a request to the Search Engine at http://localhost:8983/tridion failed with the following response: The connection can not be authenticated.**" while executing the following code:

SearchQueryData filter = new SearchQueryData();
filter.Author = new LinkToUserData() { IdRef = "tcm:0-11-65552" };
filter.ItemTypes = new ItemType[] { ItemType.Component,ItemType.Page};
XElement results = client.GetSearchResultsXml(filter);

I have checked the User is added to the SDLSearchUsers group and same has been configured in MMC Settings.