Answer
Check whether the IFSPRINT database account has access granted to the specific Company/Site that report's data belongs to — in one confirmed case, granting IFSPRINT the IFSAPP_ALL permission set, giving it the same broad company/site access the application owner has, resolved this exact symptom. The report-rendering account's own company/site access, rather than the report definition, layout, or Crystal Web Service configuration itself, was the actual missing piece, which is why only reports touching that specific company/site data were affected while every other report continued working normally.