I got this error when start a full crawl in 2010 server.
Work Arround :
Open the web.config for the website 80. Find the line : identity impersonate="true"
If the identity impersonate is set to "false" (which create the error during the crawl), set up to "true".
Then run the crawl again. It completes now sucessfully.
NB: If you have more than one WFE, make sure you updated the all servers