I restarted my application with java 7 (jdk1.7.0_75) instead of java 8 (jdk1.8.0_60) and the agent is now pushing data to my controller. But unfortunately I cannot run my application on java 7 so keeping java 7 is not an option. So it look like there is really something wrong with the agent (4.1.3.1 in this case) and/or my application and/or java 8.
Also I don't have access to the code and I'm not a programmer so adding some testing class to my application is not really an option. And I also tested the 4.1.1.1 agent and I'm having the same issue. Maybe downgrading to a 3.x agent could fix the problem?