Search has encountered a problem that prevents results from being returned

I found this error while testing a search result source in a ported SharePoint 2013 site (to SharePoint 2016): Search has encountered a problem that prevents results from being returned. If the issue persists, please contact your administrator.
When I looked through the ULS there was a number of errors relating to the result source’s query. The issue, of course, was that I hadn’t reset the search index and done a full crawl since migrating and upgrading the content database.
Problem solved!