AssertionError in com.vaadin.server.VaadinService.loadSession at line 2191

Vaadin 6 / 7 / 8
1

I am trying to start Vaadin 8.9.1 within a test

@RunWith(SpringJUnit4ClassRunner.class)
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.DEFINED_PORT)
@TestPropertySource(properties = "server.port=8000")
public class StartTest extends AbstractJUnit4SpringContextTests {
...

It starts up properly.
However, when trying to enter the application via http://127.0.0.1:8000/ I always get the following error:

...
2019-10-30 17:31:13 INFO  Vaadin4SpringServlet:85 - Using custom SystemMessagesProvider ch.sofgen.ast.controlpanel.ControlPanelApplication$1@1ee2b6c3
2019-10-30 17:31:13 INFO  Vaadin4SpringServlet:105 - Custom Vaadin4Spring servlet initialization completed
Oct 30, 2019 5:31:14 PM org.apache.catalina.core.ApplicationDispatcher invoke
SEVERE: Servlet.service() for servlet [vaadin4SpringServlet]
 threw exception
java.lang.AssertionError
	at com.vaadin.server.VaadinService.loadSession(VaadinService.java:2191)
	at com.vaadin.server.VaadinSession.getForSession(VaadinSession.java:436)
	at org.vaadin.spring.security.managed.SecurityContextVaadinRequestListener.onRequestStart(SecurityContextVaadinRequestListener.java:56)
	at org.vaadin.spring.servlet.Vaadin4SpringServletService.requestStart(Vaadin4SpringServletService.java:65)
	at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1591)
	at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:445)	
	...

Starting Vaadin with Intellij or via Spring Boot using Maven works.
I have intensively searched with Google and debugged the application but do not know how to solve this.
Any idea?
Thanks.

2

Obviously bug of Vaadin that a Java Assert statement was forgotten to remove.
Just disable Java Assertions for the VaadinSession class with

SpringBootMainClassName.class.getClassLoader().setClassAssertionStatus(VaadinSession.class.getName(), false);

Then instantiate SpringBootMainClassName directly via run method and after removing all test annotations from StartTest.
Further, do not forget to clear Java assertion status again with

SpringBootMainClassName.class.getClassLoader().clearAssertionStatus();

after Spring Boot server instantiation.