LiveCycle ES update 1 - invoke DDX with generated PDF

I've a process the receives an email (HTML Format) using the Receive operation of the EmailService.
I store the HTML body in the variable htmlDoc of type document.

In the next step I use the CreatePDF2 operation of the GeneratePDFService to convert the HTML body into PDF. The created document is stored in the variable pdfDoc of type document.

Finally I want to use the Invoke DDX operation of the AssemblerService to add some additional information to the PDF. But when I call the operation the exception below is thrown.

Where can I get the "provided" error messages.
[ALC-ASM-S00-002: Failed to execute the DDX - error messages provided.]
I used the same process at LiveCycle ES without any troubles.

ALC-DSC-005-000: com.adobe.idp.dsc.DSCNotSerializableException: Not Serializable
Caused by: ALC-DSC-003-000: com.adobe.idp.dsc.DSCInvocationException: Invocation error.
at com.adobe.idp.dsc.component.impl.DefaultPOJOInvokerImpl.invoke(DefaultPOJOInvokerImpl.jav a:152)
at com.adobe.idp.dsc.interceptor.impl.InvocationInterceptor.intercept(InvocationInterceptor. java:140)
at com.adobe.idp.dsc.interceptor.impl.RequestInterceptorChainImpl.proceed(RequestInterceptor ChainImpl.java:60)
...
Caused by: com.adobe.livecycle.assembler.client.OperationException: ALC-ASM-S00-002: Failed to execute the DDX - error messages provided.
at com.adobe.livecycle.assembler.AssemblerServiceImpl.execute(AssemblerServiceImpl.java:483)
at com.adobe.livecycle.assembler.AssemblerServiceImpl.invokeDDX(AssemblerServiceImpl.java:29 6)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at com.adobe.idp.dsc.component.impl.DefaultPOJOInvokerImpl.invoke(DefaultPOJOInvokerImpl.jav a:118)
... 123 more
Caused by: java.lang.NullPointerException
at com.adobe.idp.DocumentCache.getObject(DocumentCache.java:72)
at com.adobe.livecycle.assembler.SharedStore.hasDOM(SharedStore.java:291)
at com.adobe.livecycle.assembler.SharedStore.close(SharedStore.java:242)
at com.adobe.internal.pdfm.Document.close(Document.java:127)
at com.adobe.internal.ddxm.ddx.CollateralManager.close(CollateralManager.java:206)
at com.adobe.internal.ddxm.Executive.execute(Executive.java:317)
at com.adobe.internal.ddxm.Executive.execute(Executive.java:221)
at com.adobe.livecycle.assembler.AssemblerServiceImpl.execute(AssemblerServiceImpl.java:465)
... 129 more

at com.adobe.idp.dsc.provider.impl.base.AbstractResponseHolder.handleException(AbstractRespo nseHolder.java:136)
at com.adobe.idp.dsc.provider.impl.soap.axis.sdk.SoapSdkBindingStubUtil.deSerializeResponse( SoapSdkBindingStubUtil.java:122)
at com.adobe.idp.dsc.provider.impl.soap.axis.sdk.SoapAxisDispatcher.doSend(SoapAxisDispatche r.java:128)
at com.adobe.idp.dsc.provider.impl.base.AbstractMessageDispatcher.send(AbstractMessageDispat cher.java:57)
at com.adobe.idp.dsc.clientsdk.ServiceClient.invoke(ServiceClient.java:208)
at com.adobe.common.utils.invoke.InvokeWithProgressRunner.invokeServiceOperation(InvokeWithP rogressRunner.java:170)
at com.adobe.common.utils.invoke.InvokeWithProgressRunner.run(InvokeWithProgressRunner.java: 97)
at org.eclipse.jface.operation.ModalContext$ModalContextThread.run(ModalContext.java:113)