After upgrade to prpc 7.3.1 from 6.1 SP2, we could see below error messages in logs

LocalExceptio E CNTR0020E: EJB threw an unexpected (non-declared) exception during invocation of method "invokeEngine" on bean "BeanId(pegarules_v7_03_01_SBGPI#prbeans.jar#EngineBMT, null)". Exception data: com.pega.pegarules.pub.PRRuntimeException: Cannot handle Async Request. NULL resource type
at com.pega.pegarules.session.internal.engineinterface.etier.impl.AsyncServant.invoke(AsyncServant.java:153)
at com.pega.pegarules.session.internal.engineinterface.etier.impl.EngineImpl._invokeEngine_privact(EngineImpl.java:331)
at com.pega.pegarules.session.internal.engineinterface.etier.impl.EngineImpl.invokeEngine(EngineImpl.java:274)
at com.pega.pegarules.session.internal.engineinterface.etier.ejb.EngineBean.invokeEngine(EngineBean.java:225)
at sun.reflect.GeneratedMethodAccessor73.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:56)
at java.lang.reflect.Method.invoke(Method.java:620)
at com.pega.pegarules.internal.bootstrap.PRBootstrap.invokeMethod(PRBootstrap.java:370)

https://collaborate.pega.com/question/recurring-exception-very-few-information