It helped a lot, i guess : now the server is working online (i don't lose any ping packet through the internet) but as soon as I try to get connected, the server is shutting down.
I've been looking into the /etc/usr/local/umurmur/umurmurd.log and here is what I got :
Aug 28 20:03:14 INFO: User DUVEL authenticated - [1] DUVEL@xxx
Aug 28 20:03:14 INFO: New UDP connection port 50028 - [1] DUVEL@xxx
Aug 28 20:03:22 INFO: Connection closed by peer - [1] DUVEL@xxx
Aug 28 20:25:24 WARN: SSL: verify error:num=18:self signed certificate:depth=0:/CN=Utilisateur Mumble
Aug 28 20:25:24 INFO: User provided admin password - [1] Sfyle@xxx
Aug 28 20:25:24 INFO: User Sfyle authenticated - [1] Sfyle@xxx
Aug 28 20:25:25 INFO: New UDP connection port 58057 - [1] Sfyle@xxx
Aug 28 20:26:06 INFO: Connection closed by peer - [1] Sfyle@xxx
Aug 28 20:26:19 INFO: TERM signal. Shutting down.
The 3 first lines are written when I was with the default configuration (20:03) and the following are after the change of channels (thanks to Felix correction)
Is there another log that would be more accurate on the problem somewhere else?
Regards !