Injection problem in Spring Boot Test: "messageConverters must not be empty"

Question

I have a strange problem with a Spring Boot Test. The test itself is empty, but it does not even manage to start the application. I get an error "Failed to load ApplicationContext", which is ultimately caused by an IllegalArgumentException: "'messageConverters' must not be empty".

java.lang.IllegalStateException: Failed to load ApplicationContext
        at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:132)
        at org.springframework.test.context.support.DefaultTestContext.getApplicationContext(DefaultTestContext.java:124)
...
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'requestMappingHandlerAdapter' defined in class path resource [org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfiguration$EnableWebMvcConfiguration.class]: Invocation of init method failed; nested exception is java.lang.IllegalArgumentException: 'messageConverters' must not be empty
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1804)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:620)
...
Caused by: java.lang.IllegalArgumentException: 'messageConverters' must not be empty
        at org.springframework.util.Assert.notEmpty(Assert.java:470)
        at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodArgumentResolver.<init>(AbstractMessageConverterMethodArgumentResolver.java:98)
...

I have been able to track down this problem as only happening when I combine spring-boot-starter-data-cassandra and Keycloak (keycloak-spring-boot-starter). A test project can be found at https://www.dropbox.com/s/hs9b0ltxnd5lodg/demo.zip?dl=0

Please note that this is a stripped down version of my real application. I already found out that I can avoid the problem by removing the "@ComponentScan" annotation from my main class; however, in the real application, this is not possible for various reasons.

Can anybody explain the root cause of the problem and how to fix it?