Recommended solutions based on your search
You need to set the type property in your bean XML file.
<property name="targetType" value="some value" />
If you're using boot 1.4.x you need to modify the @springBootTest annotation to @SpringBootTest(classes = DataFlowServerConfiguration.class). 1.3.8 doesn't have this problem as well.
Solutions on the web
java.lang.IllegalStateException: Marshaller must support the class of the marshalled object