I found something weird in my debian 9 config.
I changed my hostname by editing /etc/hostname and rebooting from "myhost" to "myhost-hello".
Since this modification connecting to the machine via it's ip (I don't use WINS or other functions like that at all) shows the server not reacheable.
I said myself.. maybe it's some kind of firewall issue, so I directly connected to the machine but the machine was still not reacheable via smb:// from an ubuntu test pc.
I tried to reboot the samba daemon various times with any success..
When I switched the hostname of the target machine back to "myhost" samba returned available.
I wonder.. is there some kind of automatic configuration of the hostname that makes samba not reacheable if the hostname changes? (note again that I connect to that machine via it's ip, and I don't use the wins feature or try to connect to it via it's hostname)