Error logs on Sharepoint 2013 Search Farm:
Microsoft.Ceres.InteractionEngine.Component.FlowHandleRegistry : Exceptions occurred when evaluating the flow. Microsoft.Ceres.Evaluation.DataModel.EvaluationException: Query lookup failed on 3 attempts.
w3wp.exe: All query processing components are in 'Failed' status.
Search seems to be working. On the Search Application topology all components appear healthy.
So far I've tried, resetting the index and full crawl, clearing the configuration cache of Sharepoint, restart of search services.
The only recent changes are the creation of a Managed Metadata Service Application on a different farm which consumes Search from the Search Farm and an alternate access mapping change for the Web App on the Farm which consumes Search (but the content is crawled correctly for this web app).