It is pretty hard to find the reason why the following exception occurs:
03:49:43,336 ERROR - Step 'master>checkout>checkout-qb-com.dynatrace.diagnostics.automation' is failed.
    com.caucho.hessian.client.HessianRuntimeException: com.caucho.hessian.io.HessianProtocolException: ' ' is an unknown code
        at com.caucho.hessian.client.HessianProxy.invoke(HessianProxy.java:226)
        at com.sun.proxy.$Proxy23.queryUniqueResult(Unknown Source)
        at com.pmease.quickbuild.persistence.AgentQueryCarrier.invoke(AgentQueryCarrier.java:59)
        at com.sun.proxy.$Proxy22.uniqueResult(Unknown Source)
        at com.pmease.quickbuild.entitymanager.impl.DefaultBuildManager.getPrevious(DefaultBuildManager.java:130)
        at com.pmease.quickbuild.persistence.SessionInterceptor.invoke(SessionInterceptor.java:49)
        at com.pmease.quickbuild.model.Build.getPrevious(Build.java:869)
        at com.pmease.quickbuild.model.Build.getChangeBase(Build.java:1286)
        at com.pmease.quickbuild.repositorysupport.Repository.getChanges(Repository.java:401)
        at com.pmease.quickbuild.dependency.QuickbuildRepository$$EnhancerByCGLIB$$4847fde8.CGLIB$getChanges$45(<generated>)
        at com.pmease.quickbuild.dependency.QuickbuildRepository$$EnhancerByCGLIB$$4847fde8$$FastClassByCGLIB$$5f095607.invoke(<generated>)
        at net.sf.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:215)
        at com.pmease.quickbuild.DefaultScriptEngine$Interpolator.intercept(DefaultScriptEngine.java:261)
        at com.pmease.quickbuild.dependency.QuickbuildRepository$$EnhancerByCGLIB$$4847fde8.getChanges(<generated>)
        at com.pmease.quickbuild.repositorysupport.Repository.getChanges(Repository.java:375)
        at com.pmease.quickbuild.dependency.QuickbuildRepository$$EnhancerByCGLIB$$4847fde8.CGLIB$getChanges$46(<generated>)
        at com.pmease.quickbuild.dependency.QuickbuildRepository$$EnhancerByCGLIB$$4847fde8$$FastClassByCGLIB$$5f095607.invoke(<generated>)
        at net.sf.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:215)
        at com.pmease.quickbuild.DefaultScriptEngine$Interpolator.intercept(DefaultScriptEngine.java:261)
        at com.pmease.quickbuild.dependency.QuickbuildRepository$$EnhancerByCGLIB$$4847fde8.getChanges(<generated>)
        at com.pmease.quickbuild.repositorysupport.CheckoutStep.run(CheckoutStep.java:74)
        at com.pmease.quickbuild.repositorysupport.CheckoutStep$$EnhancerByCGLIB$$a3907b11.CGLIB$run$0(<generated>)
        at com.pmease.quickbuild.repositorysupport.CheckoutStep$$EnhancerByCGLIB$$a3907b11$$FastClassByCGLIB$$e6930ea8.invoke(<generated>)
        at net.sf.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:215)
        at com.pmease.quickbuild.DefaultScriptEngine$Interpolator.intercept(DefaultScriptEngine.java:261)
        at com.pmease.quickbuild.repositorysupport.CheckoutStep$$EnhancerByCGLIB$$a3907b11.run(<generated>)
        at com.pmease.quickbuild.stepsupport.Step.execute(Step.java:548)
        at com.pmease.quickbuild.stepsupport.StepExecutionJob.executeStepAwareJob(StepExecutionJob.java:31)
        at com.pmease.quickbuild.stepsupport.StepAwareJob.executeBuildAwareJob(StepAwareJob.java:45)
        at com.pmease.quickbuild.BuildAwareJob.execute(BuildAwareJob.java:60)
        at com.pmease.quickbuild.grid.GridJob.run(GridJob.java:106)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
        at java.util.concurrent.FutureTask.run(FutureTask.java:262)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:745)
    Caused by: com.caucho.hessian.io.HessianProtocolException: ' ' is an unknown code
        at com.caucho.hessian.client.HessianProxy.invoke(HessianProxy.java:224)
        ... 35 more

It seems this is the case when the latest build (not the latest successful) is used and the source folder did not get published in the build which should be checked out.
Please at least throw an exception will a explanatory message.