Answer
Check whether the query references a non-standard schema qualifier on IFS's own views/tables (for example a company-specific or legacy alias schema instead of IFSAPP) — Quick Reports validate and execute under the IFSAPP schema's own view definitions, so a query that works fine when run directly against a differently-schema-qualified copy of the same objects can still fail Quick Report's validation. Replacing the non-standard schema prefix with IFSAPP is a common fix for this exact symptom.