Thanks, I have changed form builder .failed file to .skipdeploy and now it works - drools-guvnor and jbpm-console.
However the old problem remains - I am still unable to retrieve a value from my human task. Instead of a String value it returns null and throws an exception:
java.lang.NullPointerException
at org.jbpm.marshalling.impl.ProtobufProcessMarshaller.unmarshallVariableValue(ProtobufProcessMarshaller.java:270)
at org.jbpm.task.utils.ContentMarshallerHelper.unmarshall(ContentMarshallerHelper.java:112)
at org.jbpm.process.workitem.wsht.GenericHTWorkItemHandler$TaskCompletedHandler.handleCompletedTask(GenericHTWorkItemHandler.java:230)
at org.jbpm.process.workitem.wsht.GenericHTWorkItemHandler$TaskCompletedHandler$1.run(GenericHTWorkItemHandler.java:209)
at java.lang.Thread.run(Thread.java:662)
So instead of receiving a String value filled in the human task form, I get a null value. At least something has improved - business process does not hang, but it finishes correctly.