Answer
Log onto the server hosting IIS for Crystal Web Service and use its own diagnostic endpoint directly from a browser on that host: http://host/directory/Ifs_ReportService.asmx?op=GetSavedPDFReport. This brings up a form letting you manually issue a Crystal report request with explicit FileName (the .rpt filename), ParameterValues (the result key, for an Operational Report), and LangCode fields — testing the report retrieval this way, directly against Crystal Web Service and bypassing the rest of the IFS print pipeline, isolates whether the actual problem is in CWS's own report lookup/configuration or somewhere further upstream in how IFS is calling it.