Answer
The Crystal .rpt layout file needs to be physically placed in the folder configured as OpenReportPath in the Crystal Web Service's own Web.config file (found on the Crystal Web Service host, typically under its virtual directory in Inetpub\wwwroot). Where exactly within that folder depends on the report type: Quick Reports go directly in the OpenReportPath folder itself, while Info Service/operational report layouts need to go inside a subfolder named after the ordering language (for example, an 'en' subfolder for English) within OpenReportPath. Placing the .rpt file in the wrong location relative to these rules is what produces the 'layout could not be found' error.