I can confirm, that something in 6.20.6 changed and causes this effect.
I tried to fix some small things in an old report (just formatting of text-fields) and kept receiving
Exception in thread "main" java.lang.NoClassDefFoundError: BothClusterReport (wrong name: BothClusterReport_809e19930caca7fe260fc1fa57f33f150339b8c81fea60cca055beffcb492d08)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:756)
at net.sf.jasperreports.engine.util.JRClassLoader.loadClass(JRClassLoader.java:378)
at net.sf.jasperreports.engine.util.JRClassLoader.loadClassFromBytes(JRClassLoader.java:302)
at net.sf.jasperreports.engine.design.JRAbstractJavaCompiler.loadClass(JRAbstractJavaCompiler.java:142)
at net.sf.jasperreports.engine.design.JRAbstractJavaCompiler.loadEvaluator(JRAbstractJavaCompiler.java:115)
at net.sf.jasperreports.engine.design.JRAbstractCompiler.createEvaluator(JRAbstractCompiler.java:433)
at net.sf.jasperreports.engine.design.JRAbstractCompiler.loadEvaluator(JRAbstractCompiler.java:408)
at net.sf.jasperreports.engine.JasperCompileManager.getEvaluator(JasperCompileManager.java:382)
at net.sf.jasperreports.engine.fill.JRFillDataset.createCalculator(JRFillDataset.java:493)
at net.sf.jasperreports.engine.fill.BaseReportFiller.<init>(BaseReportFiller.java:168)
at net.sf.jasperreports.engine.fill.JRBaseFiller.<init>(JRBaseFiller.java:272)
at net.sf.jasperreports.engine.fill.JRVerticalFiller.<init>(JRVerticalFiller.java:82)
at net.sf.jasperreports.engine.fill.JRFiller.createBandReportFiller(JRFiller.java:251)
at net.sf.jasperreports.engine.fill.JRFiller.createReportFiller(JRFiller.java:272)
at net.sf.jasperreports.engine.fill.JRFiller.fill(JRFiller.java:114)
at net.sf.jasperreports.engine.fill.JRFiller.fill(JRFiller.java:103)
at net.sf.jasperreports.engine.JasperFillManager.fill(JasperFillManager.java:530)
at net.sf.jasperreports.engine.JasperFillManager.fillReport(JasperFillManager.java:954)
at de.cenote.jasperstarter.Report.fillInternal(Report.java:338)
at de.cenote.jasperstarter.Report.fill(Report.java:282)
at de.cenote.jasperstarter.App.processReport(App.java:226)
at de.cenote.jasperstarter.App.main(App.java:109)
After some hours of fumbling about, and thinking I did something wrong, I stumbled across this thread and tried downgrading from 6.20.6 to 6.20.5.
That "fixes" the error.