Ain't soapUI grand?
I've found that a good ol' restart of soapUI always works for fixing the dreaded "Class not set for parameter: blank blank Coverage"
Then, it will usually stay functional for up to ~5 minutes ..on a good day. Rinse and repeat.
Another good idea is to compile your reports externally rather than letting soapUI try to coax jasper reports into thinking "subreport:MySubreport" is a valid URL.
Example java to compile .jrxml to .jasper:
JasperDesign jasperDesign = JRXmlLoader.load("MyReport.jrxml");
JasperReport jasperReport = JasperCompileManager.compileReport(jasperDesign);
JRSaver.saveObject(jasperReport, "MyCompiledReport.jasper");
Example of new <subreportExpression> usage in the parent jasper report:
<subreportExpression class="java.lang.String">"C:\\MyCompiledReport.jasper"</subreportExpression>