My team ran into this error on a SharePoint site during a disaster recovery drill while testing after failover and failback:
HTTP/1.1 200 OK Server: Microsoft-IIS/7.5 Connection: close
We have load balanced WFEs so we determined the offending WFE and temporarily manually routed traffic to the good WFE. Obviously, this is not the desired solution but it worked as a temporary solution to get through the DR drill.
To resolve this issue permanently, we tried detaching and reattaching the content database for the Web Application and, success! Mad props go out to Mark Rhodes for his post, Resolving a HTTP/1.1 200 OK Server: Microsoft-IIS/7.5 Error with a SharePoint 2010 Web Application, on this same issue. Thank you, sir!