Resteasy Yaml Provider is by default disabled reference at https://access.redhat.com/documentation/en-us/red_hat_jboss_enterprise_application_platform/7.1.Beta/html-single/developing_web_services_applications/#jaxrs_yaml_provider

When:
java class produces any of

@Produces("text/x-yaml")
@Produces("text/yaml")
@Produces("application/x-yaml") 

Perform check:

• it has to have defined dependencies in pom.xml like:

  <dependency>
      <groupId>org.jboss.resteasy</groupId>
      <artifactId>resteasy-yaml-provider</artifactId>
      <version>${version.org.jboss.resteasy}</version>
      <scope>provided</scope>
  </dependency>
  
  <dependency>
      <groupId>org.yaml</groupId>
      <artifactId>snakeyaml</artifactId>
      <version>${version.org.yaml.snakeyaml}</version>
  </dependency>
  

• existing file in application META-INF/services/javax.ws.rs.ext.Providers with content of YAML provider class:
  org.jboss.resteasy.plugins.providers.YamlProvider