I use quickfixj-examples-executor-1.6.0.jar as FIX 5.0 server.

Server does not respond on any client messages if there are leading zeros in int or float fields in client message, for example if there are leading zeros in sequnce number in message header (in login message or in heart beat message).
Server application outputs information about client conection on socket, but there is no any response on FIX message sent from client to server (there is no even an error message).
Leading zeros are permitted by FIX 5.0 protocol specification.