This is a really obscure error message when SharePoint search is not working on your site (thanks Microsoft). And, really, unless you know what’s going on, there’s really no way to troubleshoot this one. When you or users are trying to perform a search and you are getting the error message:
“Unknown Error. Troubleshoot issues with Windows SharePoint Services.”
Go to: Central Administration -> Operations -> Global Configuration -> alternate access mappings.
Add the your website’s website via the Add internal URLS button and set it to Internet.