CloudKit Dashboard error "Failed to execute query"

Something has caused my CloudKit queries to fail. On the dashboard I get an error message "Failed to execute query" when I try to "SORT BY" a field. The field is listed under Indexes as "sortable". For a different field, when I enter the field under "FILTER BY", and before I tap "Query", I get "No results". That field is listed under the Indexes as "queryable".

It used to work fine.

I have described this further, with screenshots at FB16114560

CloudKit Dashboard error "Failed to execute query"
 
 
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