osssearchresults.aspx no longer returns results (after creating Cloud SSA?)

Hi Experts:

Only real clue I have as to what is going on with customer's search "per-site" is a red block of text within the "Edit Result Block" stating "" and changing this to an existing Query Source such as "Local Sharepoint sites" didn't seem to help -- any call to osssearchresults.aspx returns no results. Any way short of rebuilding Search to resolve this? Customer was attempting to crawl SPO/ODfB sites, with no hybrid plumbing, and after failing with that, deleted the content source in Central Admin. Looks as though there's a "ghost" hanging around somewhere. Any advice appreciated, as always!