https://ci.wildfly.org/project.html?projectId=WF_PullRequest&buildTypeId=&tab=testDetails&testNameId=-8608833796003106928&order=TEST_STATUS_DESC&branch_WF_PullRequest=__all_branches__&itemsCount=50

Problem is this:

try (Session session = ContainerProvider.getWebSocketContainer().connectToServer(Client.class, uriUser1)) {
            //Wait until the client is initialized e.q. the OnOpen is executed
            Thread.sleep(10);
            Assert.assertTrue(queues.size() > 0);

That kind of pattern of a short sleep followed by an assert is not reliable.

This should loop checking periodically and only fail after an appropriate amount of time has passed. Unless that amount of time is relevant to what's being tested it should be a long time, 20 secs min, with TimeoutUtil.adjust used.