problem solved, but with a strange origin
when i put the :80 after the hostname , the header coming in the proxy is
hostname:80 when the beat is sending the HEAD and hostname when the beat send the data
And due to the fact that the haproxy was not waiting the hostname:port version we receive the 503.
I change the header processing in the haproxy, and this solve the problem.
Many thanks' for your help
Philippe