How to display Spring messages in Thymeleaf?

I created a Spring Boot web application that uses Thymeleaf as the template engine. I configured the MessageSource to look for messages in a subfolder:

@Bean
public MessageSource messageSource() {
    final ReloadableResourceBundleMessageSource messageSource = new ReloadableResourceBundleMessageSource();

    messageSource.setBasename("i18n/messages");
    messageSource.setFallbackToSystemLocale(false);
    messageSource.setCacheSeconds(0);

    return messageSource;
}

In this folder I created the file messages_de.properties with the content ticket.type.BUG=Fehler. In my template I try to display the text like this:

<p th:text="#{ticket.type.BUG}">BUG</p>

But when the page is rendered, I get the following:

<p>??ticket.type.BUG_de_DE??</p>

What am I missing? Do I have to configure any additional beans?