After a couple of days the fix engine stop working with the following message:
Multiple logons/connections for this session are not allowed. Closing connection from /XXXXX:11207 since session is already established from null
Then every 30sec(during the heartbeat I guess) it log the same error but with different port exemple :
Multiple logons/connections for this session are not allowed. Closing connection from /XXXXX:11208 since session is already established from null
Multiple logons/connections for this session are not allowed. Closing connection from /XXXXX:11209 since session is already established from null
Multiple logons/connections for this session are not allowed. Closing connection from /XXXXX:11210 since session is already established from null
When restarting the fix engine the client is able to connect again