I am using kamailio 3.1.2 and I have an issue with failure route. Every-time kamailio enters the failure route an internal error is triggered and kamailio sends a  500 SIP error message:  SIP/2.0 500 I'm terribly sorry, server error occurred (1/TM).
Just for testing purpose I "downgraded" my failure route configuration to a very simple one. But even so I am always getting the same error and it doesn't matter if it is a busy, a cancel, a moved temporarily...
If I don't arm the failure route I don't get any error.

Any idea what might be wrong? Is this a bug?

This is the relevant part of my configuration