SharePoint 2010, Search result error: The search request was unable to connect to the Search Service

Make sure you have connected your service to the right web application. In our case we added a new Search Service Application to replace an old one, but did not connect it to the correct web application (we had several ones). All services (computer and SharePoint) was started and crawler work fine.

In Central Administration, click Manage Web Applications and highlight your web application. Click Service Connections icon in top menu - our list of connections looked like this where our new service was "Search Service Application Work" and was not connected to our web application:

Change "deafult" to "custom" and mark all the service applications. Hit OK. Switch to the search results page and refresh - everything should work fine.